It's worth noting, IMO, that I think Buffy would've accepted Faith back in (after a time). My memory is that they were all wary but I think Buffy wanted to give her a chance once Faith came out of her coma. But Faith wasn't having it.
As for Faith going to prison...I don't think the police really had a case. Forces behind the scenes were at work. Maybe it was the Watcher's Council flexing bureaucratic muscles and other behind the scenes protocols. And the police weren't after Faith while she was protected by the Mayor, but it could be that once the Mayor was out of the picture that the cops not only stopped looking the other way but wanted to get rid of everything that had to do with the Mayor (because the Mayor scared the crap out of them and didn't want any repeats, among other possible reasons).
I suspect Buffy was cut a lot of slack (all unofficially) because the police knew Buffy made Sunnydale safer for cops (and btw, doesn't matter that Buffy was dropped as a suspect for murder, she still resisted arrest and assaulted police officers, a felony, and frankly, I think many if not most cops would be more eager to to prosecute someone who beat the crap out of a cop than someone who murdered innocents, unless of course they figured dropping the matter would save their own butts down the line later).
It's important to note that the law is not justice, and justice is not the law. It's great when law and justice coincide, but like sex & love they often don't, and can even be at cross purposes. Law is more about maintaining order so society (especially those at the top) find that life runs smoothly, and justice is a secondary concern at best. Other factors are present that are also contrary to the interest of actual justice. And that aside, it's what you can prove (or at least fake), and no way would anyone be able to prosecute Willow for the murder of Warren (though some secret government agency with their own witches and psychics and soldiers could conceivably get involved), at least not without a lot of lying (even if ironically it's because they knew the truth would never fly in a court of law). The assault on the police station, if the cops were willing to retaliate would be to shoot her dead, even if they faced conviction, because they would not brave enough to arrest a powerful "Wicca" like that after seeing her fly, shoot lightning bolts, and dissemble jails. Though if you want an explanation for it, then the witch coven in England, and perhaps even Giles, used magic and Watcher connections to get Willow out and the matter quietly dropped.
But frankly, I think a lot of it was just sloppy writing. But I know how I explain it in my head. 😛