Exactly! Thanks for elaborating on my comment.You, Sir, have hit the nail on the head! Spotify has a way round this but it wants the convenience of doing it the way it wants to do it. As you suggested this is how it could work from Spotify's perspective:
1) Create a free app (Done)
2) Offer a free tier (Done)
3) Require the user to create an account/login using their email in order to be able to use that free tier (Done and that's allowable by Apple)
4) Either during the initial on-boarding/welcome process or perhaps in the coming week or two, send an automated email to the free tier users informing them about the paid subscriptions and link to wherever you choose
Seems to me that would do the job! So why don't they do it that way? I'm guessing that the conversion rates are higher for offering direct In-App subscriptions so it seems to me that they just want to benefit off of the impulsivity of the app users rather than actually having to put in the work to build a relationship with their customers and actually show the value and benefits of signing up.