They can put it behind a clear warning, do stuff like AWS does for bucket deletion (the bucket has to be empty, you have to check a box and manually type the full name of the bucket).
There are ways to design this, they can send notifications at 60% of the threshold, 80%, 90%, 95%. They can give you a grace period, put up prominent warnings in the console and for command line tools, etc. There are ways to do it, it's far from an intractable problem.
I'm not saying that it can't happen but do you want to bet that a certain percentage of their business, for all cloud providers, is from carelessness and resources still running when they shouldn't or using more than they expected? Especially for bigger companies where it's easy to miss something. Just like gym subscriptions or other kinds of subscriptions where they're banking on you not noticing for a long time ;-)