For the record I completely agree with you that these kids are essentially employees for the university bringing in cash and notoriety on the field. I think to try a couple of points:
Schools have to maintain a certain APR for their programs/athletic departments by NCAA rules. If they admit too many of these questionable students, its going to negatively affect that score, which leads to possible sanctions or bad press. Now I'm guessing this isn't too strict when you consider what some schools that clearly do not get in trouble for this. I have a friend who ran track at ND who said that a lot of times other athletic teams try to have higher scores in an effort to beef up the entire program.
Politics-wise, I think there is a lot of power struggles between athletics and the overall school in terms of who runs the University. Along those lines, a lot of administrators could have personal agenda's or this idea that the student athlete should be just as good as any other student. I think of this as the doorman at an apartment building who won't open the door for you to visit a friend because you're friend didn't come down to the lobby, check you in, etc. There are some that are so hell-bent on feeling important and following the rules in the most strict way, even if it isn't practical for the purpose.