This is a stupid discussion which is obviously coming from those with an American Centric point of view. Bolt did not grow up with American football and likely has ZERO interest in the sport. Though it is hard for some people to believe outside of the US very few people care about football or follow the NFL. When I hear Bolt talk about other sports - I hear him talk about soccer, cricket and basketball in that order. Besides Bolt is reportedly getting ready to sign a $100 million contract for the chinese market alone (story here):
http://www.runzoom.com/story.php?title=reports---bolt-offered-us-100m-chinese-contract
I doubt he can make that in the NFL where he would likely be a second tier player.