Clarett lost on the grounds that the players union had agreed to the three year eligibility rule

The decision was a pro union holding by a three judge panel that included a "wise latina" (Sonia Sotomayor). Without the collective bargaining agreement, the NFL might have lost on an anti-trust argument. Ironically, three years is about the average NFL career... so the union was trying to make sure that its members got the maximum earnings they could reasonably expect. Otherwise, the NFL would hire as many low cost rookies as possible.