Graduation Rate is such an over-hyped statistic, and is painted with a negative stigma before justifying why people are deciding to leave school on each individual basis.
Let's stick with Football first. Like another post said, school is a diversion, yet it's a necessary one for a player to get to the NFL. If you come from a lower income situation, or even middle class on a scholarship, and you are told you can make 7 figures by leaving for the next level, in most situations you'd be an idiot NOT to take the money and run, rather than risk an injury by staying put.
It's not an insult to the school, nor a knock on the student athletes' academic aptitude, it's a real life business decision!
What is the purpose for college? To prepare students for life and to be able to make a decent enough living for yourself and family. Society as a whole shows a bitterness (or jealousy) towards athletes who go pro early, because they make a ton of money quick, whereas the normal "athletically un-gifted folk" have to spend 4-6 years for a BS and\or MBA to get to a point where they can make ends meet.
Who's keeping track of the normal working student who quits after a couple of years because they fell into a good job situation?
Some of these people, (including athletes) do come back to get degrees later on. (Michael Vick)
One final thing. Those percentages would be WAY lower for overall athletics if their sport had a lucrative professional league. Vollyball players, swimmers, etc. are staying in school because there is NO OTHER option for them like football, baseball and basketball. (Even Hockey in the Northeast) But don't think for a minute, that if there WAS such a light at the end of the tunnel for these other sports, they'd be leaving early too.
These percentages are meaningless.