My guess is that billing lags enough that they can't stick to a price cap, which means that they either have to guarantee the price cap and swallow the difference, which could be exploited by malicious users to get free compute, or they have to say that there's a delay on it which makes the cap fairly useless.

Some of these services are billed by such small increments I can't even imagine how complex billing for them is in practice. I'd be surprised if bills are eventually consistent within 24 hours.

I wouldn't be surprised if we see an announcement like billing being guaranteed after 1 hour at some point in the not too distant future, but I'd be surprised if we see realtime caps.