If I reduce my gdrive subscription they don’t simply delete what I have over the new (lower) limit. There is a grace period and it’s standard practice. Why should it be any different in this case?
I've heard that Google keeps Google Drive data around for up to two years if your subscription expired and your account is over quota. They could certainly do the same with other cloud storage.
There is, and it would cause an outage while still not achieving the supposed goal of not going over budget. You don't want to be killing your customer's production over potential misconfigurations/forgotten budgets. Especially when you'd continue to bill them for the storage and other static things like IPs.
It's so much easier for them to have support wave accidental overuses.