How do you know that it's trivial to add?
The difference between Amazon and gym subscriptions is that Amazon can easily make more money by upselling to happy customers than it can by screwing individuals or small businesses out of relatively small amounts of money for their accidental usage.
Banks like overdraft fees. Amazon can upsell to unhappy customers if the switching costs are high enough.
Hell, hard caps can be an upsell.
Before AI that is.
In cpu compute margins are high and there is a lot of excess compute, you can give some away and make money.
In the age of AI this is gone. These companies have taken on huge debts. The operating margins are going to crash as interests increase. AI enabled fraud is going to make stolen service even more common.