When you have an unsatisfactory experience with a AAA contractor, you can contact the main number and make a complaint. They follow up and get an idea of what happened and why. If it was just a bad day, then nothing much will come of it and the next time you use that service, the experience will be better. If it is a trend, the AAA is good about investigating and making modifications to make sure their customers get the best service. We are paying for it after all.
They traditionally will ask questions about the specific experiences and if they get several complaints, then they might put them on probation. Those contracts don't last forever and there are plenty of towing companies willing to take over the territory.
If they don't perform up to the standards of the AAA, and those standards are pretty high, then they're out. That's one of the hallmarks of AAA. Their high standards make it imperative that the contractors keep their customers happy and paying those yearly subscriptions. AAA has been around for a long time and they have turned customer satisfaction into an art form. If you want to play with the big dogs, then the contractors better know the best way to fetch their customers.