They don't, because of Apple's 30%. That is the point, everybody is getting the same treatment, some accept it, some do not.

Spotify is now doing the same thing Amazon does with their digital services, so why is Spotify running into trouble?

I don't know as I do not have all the facts, or any context other than what Spotify is claiming. I think they should not be in trouble if what they are doing is the same thing Amazon is. I'm also of the opinion that Apple should get rid of the rule that prevents companies from advertising other forms of payment inside the app.

I was just responding to the original comment that made an incredibly childish argument agains the owner of a store not being able to compete with products from other companies.

Really, childish? You don't have to agree, but no way was that "childish."

Ok. I don't agree. The original argument described literally how every other store (specially big chains) in the world works.