Earlier quoted context omitted.
The legal basis is that Apple is not privy to the transaction that happens outside. Purely on a data privacy basis, Apple cannot force a vendor to tell Apple what it does outside of Apple property. So any link tax that Apple wants to impose would have to be a fixed cost, and not a percentage of what happens in a different universe.
Have you ever looked into the world of retail space leasing? It’s quite common for retailers to be charged a percentage of gross sales per month. (Not defending anything here)
Sounds like literal rent seeking behavior.