> This is unlikely. The big cloud providers routinely cancel bills based on accidental usage

That is not an evidence alone. They also don't cancel bills in many cases. Most people also just swallow the damage and pay. Like gym subscriptions where people just pay because cancelling was too hard and they postpone the canceling because it takes so much effort. And they also think it is their fault of not cancelling, because it took that much of effort.

> The real explanation is just that it's technically difficult to calculate all usage in real time and shut down systems immediately as soon as a certain cap is reached.

That is trivial to add these days. Cloud providers usually say that "we care about your business and don't want to make your systems go down unexpectedly if you did not pay your bills".