McKinley is British. I don't know about the others, but I agree with your sense that most of them are American.

My first thought is that, at least in Western media fandom, more fanfiction is written about SF/F properties than those of other genres. So SF/F writers are more likely to think of fic as an issue, maybe? If you look at Fanlore's list of authors with fic policies, both pro and con, there's notably more authors associated with SF/F than otherwise. So it might be purely about numbers. I definitely agree, though, that for writers of SF/F, particularly the sort of epic fantasy that owes a serious debt to Tolkien, are in an odd position, at best, when they rage against fic.