Apple made themselves an exclusive payment provider for all iOS in-app subscriptions and payments for digital products. Merely mentioning anywhere that cheaper payment options exist is an AppStore ToS violation.
I generally support Apple but this is inexcusable. Regulators need to come down hard on this kind of payment monopoly.
If I was a businessman running a store, I definitely would NOT allow anyone that wants to sell in my store any mentions that they could be found cheaper elsewhere or get a discount somehow that could reduce my cut.