Would users care? It's not like Spotify is offering any incentive to users to sign up outside the app. Who's going to type in a URL to a website when they can just touch the subscribe button?
I don’t mean to be rude... but if opening your browser (one tap/click) and then typing “www.spotify.com” (15 keystrokes) in order to have tens of millions of songs available to you 24/7 is the hardest thing you have to do in life then you aren’t suffering any hardship at all!! You may, however, have a bad case of the “First World Problems” :/
I just tried it...an extra 8 seconds over clicking a button...are people really that time poor these days?? Because if you don’t have that extra 8 seconds (a one time thing) then you have bigger problems than the unavailability of an IAP button...