> You also don’t have to buy a subscription in the app, do you? You can pay on the web directly to Spotify and use the subscription in the app.

Apple doesn't allow App developers to Inform users about Alternate payment options.

A fact Spotify is not allowed inform you of, and is not allowed make the only way to buy a subscription

Spotify is forbidden to mention it in their app, though.

Thats a violation of freemarket

Say you buy a can of Coke at Walmart. Would Coca-Cola be allowed to print on that can that it can be bought for less at 7-Eleven?

Wallmart and Appstore are not equivalents:

I don't need to accept a huge licence agreement before I enter a supermarket

If I were to piss off their staff and get banned from Wallmart, they don't get to take away everything I bought from Wallmart in the past 10 years.

If I buy a game from Walmart, and the game sell in game items, I don't need to call wallamrt employee to come to my house to collect the money for that - that's effectively the epic lawsuit.

The way I've seen the rules inforced (though inconsistently) it would be more like walmart not letting Coke put their website on the can, because you can buy coke directly from the website.

Apple has blacklisted apps for allowing that. They're a bit selective in enforcing that rule, but that doesn't make the situation better.