1. There are good, ethical people working at these companies. If you were going to train on customer data that you had promised not to train on there would be plenty of potential whistleblowers.
2. The risk involved in training on customer data that you are contractually obliged not to train on is higher than the value you can get from that training data.
3. Every AI lab knows that the second it comes out that they trained on paying customer data saying they wouldn't, those paying customers will leave for their competitors (and sue them int the bargain.)
4. Customer data isn't actually that valuable for training! Great models come from carefully curated training data, not from just pasting in anything you can get your hands on.
Fundamentally I don't think AI labs are stupid, and training on paid customer data that they've agreed not to train on is a stupid thing to do.