I think there’s also some rules in place that say you can’t discourage people from using IAP vs. other payment methods. So a price disparity would probably not be allowed either.
YouTube apparently does this, and Apple also suggested it in their letter to Hey.
Couldn't they just charge people coming from the app store MAXINT dollars to be technically compliant (the best kind of compliant)?
They could, and I might be wrong here but Apple also disallows you to mention that a cheaper version of the service is available elsewhere on the app. So the user has no idea what the actual price is or where they could avail the price. All they see is a button that unlocks an ad-free version for $2,147,483,647 (Assuming you're using a 32bit signed int).
No there aren’t. Spotify use to offer in app subscriptions at a higher price.