^Not sure what the correlation there is. The black Nick Fury is not a creation of the movies, it's from the "Ultimate" line of comics, from which the movies have borrowed very heavily, so it does come from the source material.
Right. It's kind of a self-fulfilling prophecy; the creators of the Ultimate comics wanted to base their version of Fury on Samuel L. Jackson, and as a big comics fan, he gave them the rights to use his likeness in exchange for the first crack at playing the character if he were ever in a movie (or something like that).
Besides, there is really nothing intinsic about the modern Nick Fury that states he must be played by a white man. If they were doing the WW2 "Sgt. Fury" version of character, then race might be an issue, but as long as the character is well-written and portrayed, I wouldn't care what race of actor portrayed him in the modern context.
Sure, in real life, an African-American wouldn't have been allowed in combat in WWII, but then, in real life, a soldier from WWII wouldn't have had his aging artificially slowed so he'd still be young and vigorous in the 2010s. So one shouldn't demand absolute realism anyway.
And there is precedent, going back to the very beginning of the Nick Fury character. The Howling Commandos were shown as an integrated unit including the African-American Gabe Jones from the very first issue of Sgt. Fury and his Howling Commandos
in 1963. So in the Marvel Universe, the armed forces were integrated years sooner than they were in real life.
Hell, one of the only good parts of the Daredevil movie was casting Ving Rhames as Wilson Fisk.
Actually that was Michael Clarke Duncan.
Colorblind casting in superhero movies goes all the way back to Billy Dee Williams as Harvey Dent in Batman
(1989). Lois & Clark
cast Sherman Helmsley as the Toyman, not to mention the half-Japanese Dean Cain as Superman. Smallville
had a black Pete Ross and J'onn J'onzz and a half-Chinese Lana Lang (well, the actress if not the character). The Middleman
cast a Latina actress as Wendy Watson, who in the comics was a redhead who didn't speak a word of Spanish.
The Last Airbender, on the other hand is specifically portrayed as taking place in a culture with asian influences, asian names, and asian people. So the casting of caucasians in the starring roles is a bit more glaring in that instance, and was basically just pulled out of M. Night Shalayman's ass without any regard to the source material.
Actually I can somewhat forgive it there, since from what I read, they did sincerely try colorblind casting -- they opened it to candidates of any ethnicity, chose what they considered the best people for the lead roles, and then cast the other roles to match the leads' ethnicities. So we had a Caucasian Water Tribe but ethnically mixed Air Nomads, South Asian Fire Nation, and Asian/African Earth Kingdom. Still diverse, just differently so (and it would've become more so if there'd been sequels and Azula, Toph, etc. had come aboard). Although it's true that they would've probably had an easier time finding good Asian lead actors if they'd done their principal casting someplace other than Texas. And the actors they did cast weren't all that good, so I wonder if they really did make the best choices. It is possible there was some pressure from executives to cast white leads, but I think the worst Shyamalan deserves blame for is not pushing back hard enough against that pressure, or simply not picking very good actors. (Well, at least where the casting is concerned. The biggest thing he deserves blame for is thinking he was qualified to write the script himself.)
But really, if finding the most talented actor means changing the ethnicity of a character in an adaptation, so be it. Strict adherence to the source material shouldn't matter, because an adaptation is not supposed to be exactly like its source; it's a new work inspired by the source but telling the story in a new and distinct way. And the decisions about that work should be based on what's best for that
work. So they can take you in a different direction from the original.