It is unreasonable if they are not the one providing payment infra.
They already charge developers 100$ pa for the app store account. If that doesn't cover their costs, they can increase it, but going after 3rd party payments is pure greed.
If you think about it, a fee per app download is a lot less flexible when it comes to monetising purchases. Means free apps either become paid or shove into you a lot more micro transactions. That exact model is what made devs get mad at unity (although unity doesn’t provide the download infrastructure and it was on top of a cut)
Taxes contribute to providing services and infrastructure to the people. This takes money from the people and deposits it into the bank accounts of the wealthy. I see two very different things.