There's a difference. Hikaru Sulu and Nyota Uhura are written as coming from Earth, from real-world ethnic groups. Aang and his friends and enemies are from a fictional world with equally fictional ethnic groups with cultures that have just as many Western influences as Asian influences.Probably, because it's important. Ethnicity is not a non-issue where A:TLA is concerned. This is one point that hasn't been raised yet, I think: the fact that one of the defining qualities of A:TLA is how progressive it is in its celebration of non-Western cultures. That progressiveness, that symbolism, is important. By analogy, imagine how Star Trek fans would've reacted if J. J. Abrams had cast white actors as Sulu and Uhura. Those characters were landmarks because of the progressiveness of their casting, and that inclusion is fundamental to what ST is all about. So recasting them as white would've been rightly seen as an outrage. Well, a lot of A:TLA fans feel the same way about its inclusiveness, its status as an important step forward in the media's depiction of ethnic diversity. And as with Trek, that inclusiveness was an important part of the creators' intentions. That's why this is being reacted to as such a betrayal.
The ethnic groups inhabiting the Four Nations are no more real than Numenor or Lothlorien or Seanchan. Or for that matter Vulcan, Andoria, Bajor, etc.