Well...I think that simplifies the reality of situations like that, though. People don't speak up for a lot of reasons--fear being utmost among them. They may not necessarily *agree* with what's being done, but they're too scared to speak up or act against those in power, for fear of being victimized themselves. Not everyone is brave enough to actually have the courage of their convictions.

Which is not to say that the anti-Muggleborn prejudice isn't a real and pervasive problem, both at the Voldemort kill-them-all level and at the less aggressive but more pernicious "you're good at that for a Muggleborn" attitude that we sometimes see in, say, Slughorn.

Just like real prejudice in our world.