The film doesn't deal with boxing, but instead with Ali's battle with the Supreme Count over his belief as a Muslim as conscientious objector during the Vietnam War. Ali was actually convicted by a Houston court and spent three years exiled from the ring as his case slowly made it's way to the Supreme Couurt.
(And for those too young to remember that was a HUGE story back then; what athlete would have the guts to do something like that today? Go ahead name me one)
Personally, I agree with Frears' decision since, for me, there is no actor I think of who could do justice to Ali (And no, not even Will Smith) and having an actor to play the role would constantly take me out of the film.
What do you say?