I've always been under the impression billing is async and you really need it to be synchronous unless cost caps work as a soft limit.

You can transfer from S3 on a single instance usually as fast as the instances NIC--100Gbps+

You'd need a synchronous system that checks quotas before each request and for a lot of systems you'd also need request cancellation (imagine transferring a 5TiB file from S3 and your cap triggers at 100GiB--the server needs to be able to receive a billing violation alert in real time and cancel the request)

I imagine anything capped provided to customers already AWS just estimates and eats the loss

Obviously such a system is possible since IAM/STS mostly do this but I suspect it's a tradeoff providers are reluctant to make