1) The Big 12 did not have the automatic provisions in place. The American has a specific clause detailing what happens when specific teams leave.
2) The Big 12 did get a haircut of sorts - their contract remained the same despite the defections when they likely expected an increase.
3) Never confuse the American's place at the table. The Big 12 is one of ESPN's prime time networks - that is a partnership. The American is filler - ESPN doesn't even want most of it which is why we have to hunt for CBS Sports Net a dozen times a year.
The Big 12's contracts with ESPN and Fox were already in place before Nebraska and Colorado left.
To keep Texas from defecting to the Pac 10, the conference commissioner managed to convince ESPN and Fox to not lower the value of that contract.
Texas was also guaranteed it could start working on its network.
So as it turned out, the Big 12 (with 10 schools) was getting paid the same as if it still had 12 schools. The money that would have gone to Nebraska and Colorado went to the remaining members in the conference. Remember, this contract was for the Big 12's Tier 1 and 2 content only.
When the contract was renewed, it was for a higher amount than what the Big 12 was getting previously, with the understanding the conference needed to not lose anymore members. When A&M made the decision to leave, BYU was considered as a suitable replacement, but then TCU got the offer in the end. West Virginia was approved by ESPN and Fox as a suitable replacement for Missouri, with the expectation it would be ready to join the conference before the start of the 2012 season. As you know, WVU had to sue the BE in order to leave early. The Big 12 provided WVU with lawyer consultants and $10 million support to help with the exit fee. Starting this year, WVU will be obligated to pay back $5 million ($1 million per year for the next 5 years) with the other $5 million being considered as a gift from the conference.