Spotify gave Apple the middle finger, so Apple blocked Spotify's app update
Apple called foul. There's a clause in its developer rules that mandates apps cannot offer codes or encouragement to side-step Apple's in-app billing mechanism, so they threatened to remove the Spotify app from the store unless it complied.
This is where things get heated. In its last app update, Spotify removed mention of the promotion... but also turned off its app store billing option, completely. If you updated your Spotify app recently, you'll notice that its devoid of your credit card information, and there's absolutely no method of paying for Premium if you're a new user.
Apple said "ah hell no," and when Spotify tried to push a new update, Apple blocked its release. This prompted Spotify to fire off a sternly worded letter to Apple -- and to Congress -- alleging that the company was being anti-competitive in order to encourage sales of its own Apple Music subscriptions.