But Marvel did it for Nick Fury because Ultimate Nick Fury is Samuel L. Jackson. There is a basis for that casting.
All that tells you is that taking the Nick Fury concept and making him in the mold of SLJ worked, so it doesn't matter if it was done first in the movies or Ultimate comics.
That's what I don't like about folk claiming the films are faithful to the spirit of the comics, because they did it that way in the Ultimate universe. The U comics were meant to be an experimental series of What if type books, having fun with the concepts without being beholden to continuity or strict character conceits.
When you start adapting the films from those books, you run the risk of the movies being just as full of crappy 'new' ideas that Stan, Jack and Steve did not intend, as much as you would if you sat down and disregarded the original comics and came up with new ideas at the screenplay stage.
Having Nick Fury as SLJ is still as unfaithful to the comics as if it had been brought in at the screenplay stage to me, because you are immediately saying there is no chance of the original cigar chomping Fury being brought into the movies, therby disapointing some fans who have wanted to see him adapted into films since the 60s.
But, y'know having SamLJ as Fury works well too, which just so happened to be proven when they experimented with that concept in the What if type run of Ultimates.
So the casting of Larry Fishburne(

Fishburne, love that name), is much the same deal, except it has so far been unproven to work in any medium, but , y'know, he'll do well acting in that role.