What makes this different from government or civil service positions? What makes this different from any position when a specific amount of experience or education is required?
Bottom line, the League can hire who it wants, provided it does not violate civil rights guidelines.
Those guidelines are not being violated here; the League can allow who it wants in, period.
You want to hire underclassmen? Fine. Go buy a team in the Arena League or the CFL. The NFL has put a specific limitation in. You don't like it? Tough. There are plenty of other folks who would gladly buy your franchise (and before you bring him up, screw Al Davis.)
He also has a solid 1st and 5th Amendment civil rights case.