Apple's 30% cut(apps that can only be signed up for online are OK; but such apps are forbidden to link to the signup page in-app; either no sign-up information, or Apple-provided payment mechanism only); which more or less assures that they'll be able to undercut their competitors on iOS, unless some miracle has made the labels 30% or more more generous in their dealings with that competitor.
When I setup my Amazon/Audible account, I did it on a web page (even if it was on iOS), and in fact, that's where I continue to make my purchases (the webpage simply directs back to the app once I've bought a book). It's fairly easy. With Spotify, it's even less of an issue - the user only has to do this transaction once. Arguably, Spotify should simply force users to do what Amazon/Audible do - transact outside of the App for purchases.
Why should Apple be forced to help Spotify make their App simpler piggybacking on the simplicity of iOS without renumeration? Allowing purchases within an iOS app is not some sovereign right of an app creator. It's Apple's rules, and is not dissimilar from Google's rules (even the % is the same - 30%).
Finally, Spotify is owned (20% from last accounting)  by the music industry - it's their (very successful) stalking horse into streaming music, which is partly why they've been so successful - Spotify got deals/labels that other apps (Pandora, LastFM) simply could not. It's quite ironic that Spotify is complaining that they can't compete -they've been the ones benefiting from an unlevel playing field for years.