My guess is that, in this case, "too big" actually means "above the fraud team on the org chart." They could presumably pull the rug out from under fraudsters, at relatively little cost. Perhaps by making some policy changes that alter the cost/benefit tradeoff for fraud, or that limit the blast radius so that it doesn't render he entire platform untrustworthy. But I'm guessing the changes in question would be tantamount to an attack on some executive's golden calf.

I could hazard a few specific ideas in Amazon's case, but really I'm just extrapolating from the fact that I've seen that this is how it generally works at any sufficiently large company.