Why are they using any language at all? Why not just let the app maker open the payment screen directly?
I think there's a valid reason to think "if it's OK and common for banks and brokers to do it, it's OK for me to do it" and also to think "this will help protect users from being scammed by other apps who might pop open random links without any notices".
Now that I think about it, I wonder how much of the current backlash against remote work is to avoid this exact situation. Face to face conversations don't end up in evidence. Written conversations do, and video chats are increasingly being summarized and recorded by AI.
The fig leaf reason is “it’s an app we approved and we need to protect our customers.”
From a more negative point of view, so Apple knows how much it happens and gets to have some influence over it.
There are a lot of websites that give you a warning when you click on a link outside of their control.