The circuit breaker doesn’t need to be 100% accurate. The detection just needs to be quick enough that the excess operating cost incurred by the delay is negligible for Amazon. That shouldn’t really be rocket science.
Implementing that functionality across AWS would cost orders of magnitude more than just simply refunding random $100k charges.
Sure, not providing that customer-friendly feature bestows them higher profits, but that’s exactly the criticism.
If it were highly profitable for them to implement some form of budget cap cutoffs, they would! It's obvious it's not a game they are interested in.