There's an essay in Buffy the Vampire Slayer and Philosophy about this. And it really is problematic. I wouldn't lock it down to their treatment of Spike - I think it is in his case not just because he is a vampire. That may be the superficial reason, but in Xander's and Giles's case - who have the most ardent objections, I think - their dislike is more motivated by jealousy and protective instincts. However the way they treat demons in general can be seen as racist. Especially compared with what we learn from Angel. The problem is naturally that we don't know in how far those things actually can be compared - but if you look at Harmony, who doesn't have a soul nor a chip nor anything that makes her different from ordinary vampires, but who is trying to reform herself somehow - Buffy would just have killed her, like she killed all the vampires.