Because they don't have anything in his contract like the following one Mike Leach (Texas Tech) had:

"The parties agree that, although this agreement is sports related, the primary purpose of the University and this agreement is educative. Thus, the educative purposes of the University shall have priority in the various provisions of this Agreement. Coach will follow all applicable University policies and procedures. Coach shall not, either directly or indirectly, breach or countenance to the breach by any player or coach subject to his control or supervision of any of the rules and standards of the Big 12 Conference, the NCAA, youth, collegiate, and master's amateur athletics as well as other associations or agencies to which the University adheres."

Analysis follows:

1.  Can anyone imagine that Cheat Pete's contract states that the primary purpose of the school and of his contract is "educative"?  Trojan football "education", such as it is, is outsourced to Senora Ross at Trade Tech.

2.  Does anyone believe that educative purposes of the school have priority over the football-related provisions?  Let's see, this is a contract paying Cheat Pete more money than any other private school employee in the country, including far more than the USC President.

3.  I do believe that he does follow all applicable University policies and procedures, particularly the one that requires all football program scandals to not be talked about and to be referred to the "compliance" department to be stonewalled for life of coach plus 20 years.

4.  Most notably, would we expect Cheat Pete to neither breach, nor countenance breach by players, of NCAA rules?  Um, Bush, Jarrett, McKnight, and all of the other scandals Menelaus has on a dedicated server.

So my best guess is that Carroll's contract includes the following controlling clause: "The primary purpose of the University is football.  Thus the pigskin purposes of the University shall have priority over academics, morality, and compliance with state and federal criminal laws.  Coach is free to breach, to have players breach, to have agents breach, and to have baby mama bosses breach, any and all NCAA rules and standards, so long as we stop losing at home to Stanford."