You also have to consider the market value of their reputations jointly as well. It would have to be a huge incentive to risk their reputation, both apples with their security conscious customers and customers with high regulatory burden, and the auditor whose only asset of value is their reputation. Auditors typically poof out of existence (Anderson anyone?)
The cynical view, of course, is that Apple's incentive and the Third Party's incentive can become very much aligned for the right amount of money.