From InsideHigerEd.com:
A group of economics and law professors has urged the Justice Department to investigate whether the Bowl Championship Series violates federal antitrust law, The Wall Street Journal reported. The letter to the department's antitrust division, which was signed by 21 professors and lawyers, asks the agency to find college football's current mechanism for determining a national champion to be a cartel that favors BCS members over other college teams.
The NCAA has already been shown to be bound by US antitrust law when it comes to the negotiation of media rights (the NCAA v. the Board of Regents of the University of Oklahoma). But this is a different animal: the determination of a champion. A league needs to determine rules that define the champion and this must be done collectively. Moreover, defining thiese rules is not in and of itself a restriction of competition. I am not a lawyer, but it seems the single entity defense may apply.
But if the league - in this case the NCAA - is restricting access to the championship, then my untrained legal mind suggests that the single entity defense is not a sufficient defense in this case. Yes, collective action is needed to set championship rules, but those rules should apply to all league members, not just the most popular one.