Yes they do. This is why Spotify is more expensive if you subscribe through the app -- because they cannot take payment details through the app for digital goods/services.

So any music streaming service that wants to try to compete with Apple music has to pay a tithe to Apple of 30% or miss out on half of America (or make customers jump through hoops).

The 'you dont have to use it' comment is moot -- iOS accounts for millions upon millions of people; you generally need to use it.