Amex doesn't care if all of your charges come from stores that withold L3 data. They will still clawback and send you a nice letter saying these all transactions aren't eligible for rewards. They have velocity triggers in place (if they choose to use them) that don't depend on whether they have L3 data or not.

To answer OPs question. They're not going after someone that buys $500 a month in closed-loop GCs. They're going after flagrant gaming in the range of at least several thousand/week to tens of thousands/month during limited-time promos.