I'm not really into Avatar, but I showed this casting to some of my friends who are and it seems Ian Ousley who portrays Sokka is caucasian. So this does mean they are still whitewashing part of the cast, despite DiMartino and Konietzko claiming they weren't going to do that. One of my friends also said they might have cast Ousley because he appears like he could be racially ambiguous despite the fact that he is, in fact, white.
Cause they aren't asian. Especially not goku.
Bulma's and Goku's cases are different since one character is human, and one is a humanoid alien. So when someone says Goku isn't Asian what they really mean - I guess - is that Goku isn't supposed to have the appearance of an Asian person, which is still something I disagree with.
In Bulma's case, having her cast by a white actress in a live action adaptation makes sense since she's from the western world in DBZ's version of earth. Apparently Toriyama even designed her to have the appearance of a "western girl".
Goku on the other hand was discovered by Gohan in the eastern world of the earth and despite the fact that he is an alien, he's a humanoid alien who has the appearance of a human. The latter is the same exact description that can be used to describe Superman, but as far as I can tell Superman has almost always if not
always been cast as a white man, so as to be portrayed with the appearance of a white human man. The creators of Superman were presumably white and American, so it makes sense that that's how they envisioned their character to appear live action.
As others have said, Goku is based on Son Wukong from Journey to the West, which is a story of East Asian origin. The creator of DBZ is also East Asian and DBZ itself is also a work of East Asia so it makes perfect sense to cast Goku as Asian in a live action adaptation and it's why I would argue that Justin Chatwin's casting was another instance of whitewashing.
DBZ: Light of Hope got the casting right. 17, 18, and Bulma were all played by white actors while Gohan and Trunks were played by Asian actors.