The general population of students all aspire to be A students at ND. The reality is the football players in question just need to be C students to get to the finish line at ND and that diploma with a C average will be worth millions more than the juco degree at any average.
Two, some kids go juco becuase they weren't star players early enough in high school. They couldn't get a scholarship to the school they wanted or maybe had no real offers at all. They can't afford even the state schools, so they go the juco route, can make that football team and two years later prove to have division I talent. That's probably under 20% of the total pool of juco players, but that still leaves 100's per year that could fill 1 or 2 spots for ND.