Nowadays quotas give you some safety net. For example you usually have to request more than one GPU to avoid burning money that way, or more than say 32 instances. It should not be possible for a new account to spawn 1k VMs overnight.
The problem with billing is that often these charges are not calculated instantly, and others are not trivial to deal with. For example what happens if you go over budget on bandwidth or bucket storage, but still within quota? What do you kill? Do you immediately shut down everything? Do you lose data? There are lots of edge cases.
You can normally write your own hooks to monitor billing alerts and take action appropriately.
The problem with billing is that often these charges are not calculated instantly, and others are not trivial to deal with. For example what happens if you go over budget on bandwidth or bucket storage, but still within quota? What do you kill? Do you immediately shut down everything? Do you lose data? There are lots of edge cases.
You can normally write your own hooks to monitor billing alerts and take action appropriately.