Not that Apple would want Spotify in the store anyway, with it being an iTunes competitor.
With OSX it's a little different, as there are alternative channels for installing Spotify, which prevents OSX from becoming "a platform without Spotify", and also allows Apple to ignore them entirely, and push their own product in the store.
There are hundreds of apps that allow you to use subscriptions made outside of the App Store. The rules for digital goods and digital subscriptions are:
* If you want the actual payment/checkout to happen ON the phone THROUGH the app, it MUST be an In-App Purchase (subject to 30% cut). * Appendix: redirecting to a website for the payment is forbidden, because it would be a too easy workaround to the previous rule. * If you want the app to just connect and use an existing subscription made elsewhere, go ahead, nothing is owed to Apple.
Dropbox for instance happily works with the subscription made on the website. A couple of years ago, they ALSO added a way to subscribe to their paid plan through the app, in which case the 30% cut is paid to Apple. I can see they doing it because probably they don't want to lose customers that feel like upgrading with a single tap is much easier than going to the website and inserting credit card details. As everybody in marketing knows, when you're potentially one tap away from buying something, the sole idea of having to go to the computer and taking the wallet is enough for a turndown for most people; impulsive buying is a strong force.
I didn't think Apple allowed this, but they are doing it.