"Why should an aspiring pro athlete be de-facto required / expected to be scholastically proficient?"
A vast majority will not succeed in their venture and these are schools. Poor career paths aside, the rest of the world expects you to be able to read and comprehend the menu in the food court. This goes doubly so if the aspiring athlete is working the till, which they often are before and after their career path.
You are correct that the vast majority of collegiate athletes will not go on to play professionally. However most of them are well aware of this and actually can read and do take their academics more seriously. There are countless numbers of people who were athletes in college and who go on to success in professions other than sports.
The truly elite athletes who are very likely going to go on to play professionally also know this and may very well not have any intention of completing a degree.
I doubt those "between 8% and 10% of the school's football and basketball players" who "are reading below a third-grade level" are getting much education. At best, they are getting an empty degree.
A vast majority will not succeed in their venture and these are schools. Poor career paths aside, the rest of the world expects you to be able to read and comprehend the menu in the food court. This goes doubly so if the aspiring athlete is working the till, which they often are before and after their career path.