Apple will approve apps that prevent sign-up in the app. The problem is that they will deny you the ability to even tell the customers where to go to sign up. Notice that the message displayed in the Spotify app doesn't have a link, doesn't even mention that you can sign up on their website. The customer has to infer that that is what's going on -- good on Spotify for using "premium" as a trigger word because Apple rejects apps that contain the words "purchase" and "subscription" _anywhere_ in your app if you're not using IAP. We were rejected once because those words appeared in an error message sent from the server.

I don’t think it’s special for Spotify. It comes under the “Reader” apps clause (originally carved out for Kindle?) where apps which sell: music, movies, books, email can require that a user create an account on the web first (but cannot link to it, which is stupid)