Big difference between saying "it would be better this way" and "this is what i would personally prefer"
Fanfic culture is largely the latter
And then there's some that's not even "this is what I would personally prefer," but more "this alternate option is interesting to explore"
Personally I'm not that interested in shipping, non-canon-compliant fanfiction, etc, but they're really not hurting anything.
Also people can make headcanons while still respecting the original material, it's done all the time.
Yeah I think headcanons by definition don't contradict canon? When they do then they're not headcanons
(I spent too long reading about that because of some stupid argument on Avatar wiki ahaha)