From what I can gather most require a minimum GPA be maintained - mind you I have also heard that everything possible is done to ensure that star players are retained regardless.
It’s a stark contrast to the Australian system. In the degree that I am in, each of our subjects has an expected work load of 12-15 hrs a week plus contact time across four subjects (full time subject load) and the mean mark is a pass (50-60%) - there is no way that you could have an athletics programme running within that type of system and our retention rate for Juniors reflect this, so at least the US system is keeping talent in the sport they play and rewarding them. I agree with you that there their balance between these two aspects is unlikely to be optimal from a societal perspective.
The course load for the quarterly system for on-time graduation is 15 units, minimum for NCAA eligibility is 12, so it’s reasonable to expect the University to guarantee funding through to graduation at that rate.