Pay-for-what-you-use pricing has the ability to escalate. You don't buy a server for $30k, and go through whatever procurement paperwork is set up for that. You just increase average usage by here and there. There's no procurement hurdle, so it can be a blank check.

..just like it's easier to get I to debt with a credit card than a personal loan. Credit card debt mounts one purchase at a time. Personal loans require you to decide to borrow €X.