Maybe because it's paid monthly no matter what, as opposed to yearly plans?
> You will be invoiced monthly based on the month’s install data. Invoicing will be the same method as for your Unity plan subscriptions, though it will be monthly regardless of your Unity plan payment cycle.
I'm assuming that $0.20 is paid every time it's downloaded based on their wording, so that could introduce a new way to harm competitors. Buy their game and uninstall / reinstall on loop?
> What is the Unity Runtime Fee?
> (...) Creators only pay once per download.
https://unity.com/pricing-updates