Assuming, for even a MOMENT that this plane of existence operates at least similarly to the way that we do, and Damacians and Noxians breath air, eat food, and even KINDA live on a planet similar to ours, Faithfeather, then there is no way that a planet like that could exists with the temperate and tropical climate without people that are darker than white folks dipped in tea. Saying "why no Americans" smacks of bigotry. America is NOT a race, neither is Asia, it's a place, containing people. If I'm to believe also that riot truly has no people of ANY racial makeup other than white or Asian, then that is just narrow minded, lazy ass design.
For those champs pointed out as black, or even tan, the progression of skins for their model shows a relatively blatant "whitening" of them. Whether intentional or not, it is a subconscious desire to make those champs and skins more desirable (putting out a product that you think no one will purchase is just silly, so attempting to make it appeal to people is the primary focus, especially if they already own the champ.
I think the question "why no black/dark skinned champs" is both very valid and something that should at least be looked at. I don't write code for games or run a business, but i sure as hell play games a buy stuff, as do most of us. I'd like to see this addressed in some form.