The original plan was for DD was for Cuba Gooding Jr to be DD. I don't know if it's the same producers that ended up doing the movie that got made with MCD as Kingpin but if it is, then there could be an argument that having proheminent African-American actors in the production was their intent from the get-go above "finding the best person possible" like you said.
Well people are reacting now because it's a pretty recent thing.MCD as Kingpin, Idris Elba as Heimdall, Tyler Perry as Alex Cross, etc. Hollywood's shown its willing to consider black actors in traditionally white roles for awhile now. This is old news, so its a little late in the game to still be getting upset over it.