Ironically, Spike is perhaps the best representation of an abusive, dangerous partner, doing better than Pete or Warren. If abusers were so obvious as to be like Warren and Pete then there wouldn't be all the questions of why women (and sometimes men) stay. Many batterers will express admiration for the women they abuse (especially at first) and make them feel like she has the power rather than him. They can say the same things Spike said to Buffy about how she makes him feel and it's all fuzzy at the time, and when it gets violent or scary, then it's that their love is hot and passionate. Eminem says something very similar to Spike in his song Love The Way You Lie (which I consider the perfect Spuffy song, btw), and plenty of batterers can express what is probably genuine remorse and guilt, only to do it again later. Sometimes he performs acts of penance, but god help her if that's not enough to prove she should stay with him.
And not all women are dependent upon such men financially, as some make more money than the men who abuse them. Most to all western nations make it relatively easy to leave an abuser if it's money that's the problem (not to say it's a magic carpet ride or that he can't use the same system to harass and even terrorize her in response, but plenty of organizations will help women trying to leave if she can't get a lawyer, and if he's dominating her through finances then he can be forced to pay if they're already married through a system that makes it even easy for women not abused and not poor to get big payments).
It comes down to they think it's an isolated incident and that he's sweet who even respects her (most of the time), even when 1 time turns into 20, because of how he seems to love her so, a feeling that very well may be sincere (those whom we love the most can also hurt us the most and the easiest, and the instinctive response is to hurt back in one way or another, so combined with poor impulse control and poor anger management I can see why there's so much domestic violence).
And in actuality I love my mom and dad, and they violently attacked each other and me (though rare for me as I knew when to get out of the way, but as a kid I gave tours to my friends that described how certain dents in the wall got to be there from the exchanged violence between my parents), though I do think they did the right thing in divorcing and I'll never leave myself vulnerable to them. Hell, I think my dad is still a better man than Spike ever was, including season 7, despite all his flaws, though seeing Spike and Buffy exchange punches does remind me of the violence he and Mom inflicted on each other. (Note, I said a "better man than Spike," which should not be construed to say "a good man" even if I do see redeeming features in my dad rather than as some simplistic brute like Pete from Buffy.)
Btw, I've heard one share of how her ex put a gun into her face but he put it away and hugged her before leaving when she cried, something I remember when I see the scene where Spike visits a crying Buffy while he's holding a shotgun.
Another reason is what often happens when she does leave. In some cases the stalking (such stalkers can probably relate to Spike in his stalking phase), assaults, and violence happens (up to murder, which may include more than the woman who finally left him, including the kids) It can lead to situations like in Seeing Red. When my mom left Dad (though she did provoke him out of spite and legal strategy as opposed to she just left), I spent some time crying under my bed one day (when I was 13) because I was sure Dad was going to kill Mom and probably me, too (and maybe he would've had the cops not intervened). As said on Buffy, love makes you do the wacky.
But it's not just that, but how he can threaten to harm or kill himself, and she may still love him despite all the abuse, essentially holding himself hostage (hell, one guy who wasn't abusing me beyond taking advantage of my generosity tried to make me continue pandering to him by implying he'd kill himself if I stopped). Spike does that as well, even after he accepts (unlike many in the audience) that he can't actually "win" her. And sometimes it works, and as many will warn her not to fall for it she'll keep it secret as much as she can (as Buffy did with Spike in both seasons 6 and 7, and even sided with him just as many battered women will side with her abuser against her own family and friends and even outside legal authorities that try to intervene whether or not she wants them to). But all too often that means the cyle of abuse will continue, despite the persistent (and pernicious) fantasy otherwise.
If Marti wanted to make a feminist message rather than an all too common love story (in fiction) then she could've capitalized on this.
