Apple is not "actively trying to prevent" people from managing their Spotify, Netflix et al subscriptions outside of their respective iOS apps, what are you talking about?

The app literally can't send you to the Web page so you can sign up there. It has to say something vague for Apple to approve it for publishing. Here's an example from Kobo [1]. They apparently can't even tell you the domain name.

[1] http://imgs.fyi/img/7hut.png

And how does that actively prevent people from using Spotify on their desktop?

Uh, Apple rejects your app if your app talks about being able to subscribe/upgrade via means other than IAP (includes desktop)

How are Apple trying to prevent people using Spotify or Netflix on smart TVs and Linux desktops?

I suspect spiderfarmer meant that Apple is trying to prevent "[Using] spotify and netflix without going through apple", at least on iOS.

How? I use Netflix on my iPad all the time and not once has Apple tried to stop me.

Netflix is not allowed to point people to their website from their App store description, which is absurd to me.