I did. They don't.

They offer billing alerts, have a budget tracker thingy, but have no actual automated caps. Closest thing you can do is write one yourself using the AWS APIs.

Yeah, gotta monitor daily

That is only for automated reporting and alerts. It does not allow you to actually limit your expenses.

IIRC, it can fire off to a SNS topic; you can burn down your stuff to your heart's content.

Billing alerts that are simply a notification you exceeded $X is not any sort of limit, particularly when they may take hours to arrive.