Not just free money. I'm pretty sure the lions' share of any streaming service's income is from users that are subscribed but don't consume everything for that month. Their business model relies on this.
If Netflix wasn’t relying on a degree of inactivity with in their infrastructure then they wouldn’t have needed to lower the bit rates.
It makes sense, when you think about it. Over provisioning is a common practice when dealing with expensive finite resources. For example ISPs have been doing this for decades, offering households higher individual bandwidth than is available if every household within a local radius was to fully max out their throughput. VMWare also offers this to allow individual VM to consume more RAM than the total available on the host.
The key is not to over provision so much that it becomes noticeable under “normal spikes” — and I think we can all agree that COVID was anything but normal.