Or uses stripe which hides your real card number from the merchant. The annoyance of adding a card is one thing but fear of an app developer getting hacked and exposing your card doesn't strike me as a very strong reason.

they shouldnt be storing it anyways. thats not PCI compliant.

Dev logic, not how most of us are thinking

As an app developer, most users also seem to think developers are to blame for issues like the App Store failing to download apps, double-billing etc, so I don't think buying though Apple makes much of a difference there.

That's flawed logic. Your users are telling you they can't tell apart merchant/platform so you should build on top of that and assume they won't know that you're not PayPal