I am responsible for maintaining an app for a client who has an Apple Enterprise certificate. I am a contractor so I realize that I am allowed to use the client's certificate to install the app on one or more of my devices while developing and testing. Should I need more testers, I am authorized to get other users to test the app, subject to them signing NDAs and having them under my direct supervision.

That is the extent of my allowed use of that certificate. Anyone with any sort of ethics at Facebook/Google should have realized the same. Passing out gift cards and calling users "contractors" is against the spirit and letter of the contract.