Only on the internet.
Offhand, it seems only Spike and Buffy normally get a pass for anything and everything, though not necessarily together, one can fanatically uphold one and possibly loathe the other. (I remember being called a Spuffy once because I dared criticize something Buffy did, and I'm one of those that think Spike should be staked camp, though I will defend his actions and statements at times.)
By "normally" I mean when it happens, not that fans as a whole normally do this.
I think the few fanatics who will excuse Buffy anything probably just project themselves into her, an idealized version of their own self, and likely are hyperdefensive about themselves, too. But that's just a guess.
Willow does get an incredible pass by some (I can think of only one I'd call a "fanatic" about it, however), but she's also just as likely to be despised, like some of the favorite whipping boys. (For that matter, Spike is a favorite whipping boy for some with all the intensity of a pro-Spike fanatic as well. I think the Spike fanatics exacerbate that, likely in a vicious circle, but I don't think that's all it is.)
Read the rest only if you're genuinely curious about why Spike has many willing to excuse everything he does (even to the point of blaming Buffy for his faults) rather than just complaining that it's so. Not that I provide answers, just observations and musings.
I think it's worth keeping in mind that Spike is something of a common trope. Both Thor and Loki are hot and yet Loki gets far more love by men and women both (even despite having his butt handed to him on several occasions, just as Spike does).
I could go on about many series that made major bank, 50 Shades of Grey just one of the better known examples.
Perhaps this is because many mistake obsession for true love (I know this is true in at least a few cases, and one wrote she was mad at Buffy for "keeping Spike away from me" which meant she projected herself into Buffy, possibly because both Buffy and this fan had been abandoned by their fathers), maybe the bad boy can be more trusted (so they think) for the good sex without being judgmental about it (if the bad boys and girls violate social norms in society, then there's a good chance they do so in the bedroom as well).
And I can understand that it feels more real than the more idealized versions of romance (which border on, or are, BS in real life--though so are the tropes of Beauty taming the Beast as in later Spuffy).
There's also the disturbing phenomenon of how we as a society say we don't like psychopaths, sociopaths, and narcissists, and yet keep finding them hot (not just romance, also in the corporate and political fields--the latter by popular vote, and I don't mean to limit that to any one specific party or ideology, meaning the MAJORITY of people find that type of person appealing, no matter what they say, and often over identify with them, excusing them anything).
That is Spike isn't an anomaly, he's just another manifestation of something very common in the social psyche.
There's a feminist blog (don't know if it's still active and too lazy to look up that needle in the haystack) in which the writer openly stated, to her own discomfort, that as much as she criticized Xander's toxic masculinity, she actually found Hyena Xander hot, and she doesn't know why (and wishes she didn't, but has the honesty to say what she felt).
I do wish this was explored more. I think humanity needs to understand itself more if we're to survive as a species, or at least stop whining about what we always get generation after generation despite endless attempts to change it. (Perhaps it's evolutionary psychology in which the bad kinds we crave were once better equipped to survive a brutal life, but now in a society with rules we're conflicted, as the rules have changed the ancient instincts remain...and I agree with what we resist persists. Like in the Bible Belt where preachers constantly said even THINKING of sex was a sin, so what else are they going to think of when bored, especially when its a hardwired instinct anyway?)
For whatever reason, people crave people like Spike, whether they want to be him, or be his.
I don't mean everyone (certainly not me), of course, but enough that it would be foolish from a marketing perspective to ban characters like him from a story for being too problematic.
Besides, entertainment is generally supposed to be an escape from our lives, not another window of being preached at or bored with sanitized stories (and though many political and religious forces, Left and Right, have always tried to censor all forms of media, they never have more than limited and/or temporary success).
Though I can't help but be reminded of the tongue in cheek "evil attracts women like death attracts flies" in a fun song, I've noticed many men also favor bad girls (as well as non-apologetic butt kickers) over wholesome ones...even crazy ones, like after the movie Serenity came out, I saw a big discussion by men who suddenly wanted to marry the character despite how insane she was. The insane, psychopathic yandere female character of anime appeals to many guys as well despite that in real life it would be a horror story. (This isn't about the petty, vengeful types, that is the female Snyders.)
I've known men and women who preferred Faith to Buffy (though not in overwhelming numbers), at least in some ways (I think part of it is that Faith was unapologetic and "cried less" because her popularity waned with those types on Angel).
I think the Aunties on Chilling Adventures of Sabrina were the most popular version to ever appear (and about stole the show from Sabrina herself), and there's a long tradition of it. (For that matter, I gather Riverdale could provide more examples of loving the bad girls, and how those who would sanitize our entertainment are doomed to frustration.)
Interesting enough, there was me and another girl who played with boys when we were all under 10, so not much for crushes and the like. We were playing Buck Rodgers, though I did not know anything about that franchise other than what they told me that day (and still haven't seen or read anything about it to this day), and as girls we were expected to choose one of the female characters, and given 2 choices.
The other girl asked "which is prettier?" Turned out it was was the villainess, and so she took that character's role, even though it put her with the bad guys (I personally did not care and did not contest her choice). Even at that age the boys found bad girls thrilling, and I doubt it's because of the deliberate intent to make the bad girls appear more sexual (as they don't always, some can be ice queens that cover everything up, and still get the love, and I certainly don't think we as children were thinking in those terms--I wasn't, anyway).
Why? I don't know. Many don't know who wish they did as much I do. But it's undeniable, and thus what you describe with Spike does not surprise me at all.
Heck, I loved him, too, at first, not that I wanted to ship him (and I did want to see him dusted eventually, though as he was such an entertaining villain, I wanted it to be appropriately epic, applauding the character).
And it's apparent to me that the majority of Buffy fans, and that of most franchises, are shippers (not to imply just "bad boy/girl" shipping), and while there are many Spuffies who acknowledge the problems of Spike (and Spuffy), I can't recall any who fanatically defended Spike everything who wasn't a Spuffy (though that doesn't mean there isn't one, who actually thinks Buffy wasn't good enough for Spike, gods that would make me laugh, though such a person probably ships Spike with someone else).
