I agree that slash is also quite controversial, but from a personal perspective... AU comes with a lot of controversy.
My fanfic "career" started with writing only AU, I didn't really get noticed until the first canon story I wrote. But I will say that as someone who prefers writing, and reading AU, it comes with a lot of opinions from those who prefer canon.
Many times I've been told that the things I've written aren't fanfic, might as well be OF, etc because the boys aren't Backstreet Boys in them and sometimes there's only an appearance by one character. In my mind though, when Nick gets cancer, or Brian and AJ fall in love, or Howie gets addicted to meth, whether they're Backstreet Boys at the time or not I don't think it's canon, that's a universe alternate to the one we live in, no different than if he weren't a performer and were a janitor instead. And if we all only wrote specifically to canon, to the universe that we live in right now, we'd all be writing the same tour/recording story.
I think it's about keeping the characterizati on the same more than keeping professions, etc the same.