I would guess it was part of the contract they signed with the NCAA a couple years ago. ESPN was on the cusp of buying all the rights, when CBS and TBS teamed up to air the tournament on four channels simultaneously. This deal was only possible because TBS provided three channels (TBS, TNT, and TruTV), and I would guess that a condition of their participation was that they get the Final Four. It's still a good deal for CBS, as they get part of the tournament ad revenue instead of none, and a great deal for TBS.