The times when "every able bodied male could count on being drafted" were quite rare, and at those times it was generally possible for most well-connected people (not just entertainers) to find a substitute for going out and being shot at. For example, during the height of the Vietnam draft, professional baseball players could get into relatively safe and cushy reserve units - somewhat like Bush43 did when he was of that age.
Counter-examples are also plentiful: Ted Williams, Warren Spahn, and many other standout major leaguers willingly gave up years of their careers to serve proudly during WWII and Korea. Jimmy Stewart. And on and on.
The Cassius Clay case is one that could fill a couple of interesting books. He claimed, in fact, to be a "Muslim minister," and a pacifist - both of which were preposterous, but he got away with it, didn't he? He was led around by the nose by a couple of Nation of Islam hacks, but he was both a dimbulb and amazingly gullible.
Little known was the change in the draft that allowed him to shift from 4-F (unsuitable) to 1-A.
The military service had an aptitude test that was scored on a percentile basis (the "AFQT"). For a long time, they would not draft anyone whose score was below 15%. At the height of the fighting in Vietnam, they lowered it to 6%. Cassius Clay's score was 7%, thus he became eligible. Draftees under this program got a unique service number starting with "US67..." They were the dumbest collection of misfits imaginable, and invariably were given jobs as security guards and mess hall workers. Clay's supporters claimed that he intentionally screwed up the test so that he wouldn't be drafted. Decide for yourself whether this is true or not. I think it was his "real" score.