Genetically, Spike is a human with a little vampire in him. And Buffy's technically human too, so I think there can be a little leeway. He can still feed off people if they let him. He just can't physically hurt them knowing he's going to hurt them. Like, he pointed a gun at Xander and wound up in pain because he wanted to pull the trigger. So it's really about intent. Spike is a sad puppy, man.

But he fed off a dead girl and I'm still having to finish my rewatch, but I think he can feed if the person's into it and he's not intending to kill them (or maybe he can turn them, if they want it? idk. that's shady territory). So Cain could like, test him. Or give him blood. Or they could get in a fight. Or Spike could lick it off. Idk. He's weird.