I think the parent was implying that they advertise for Spotify in the same way that your local grocery advertises bags of Doritos, not that there is some explicit advertising relationship going on.
They're not entitled, and neither is Spotify entitled to sell on the app store on terms they want. It's just a business transaction. It either works for both sides, or it doesn't.
It's understandable that Spotify doesn't want to pay that cut, and they can try what they're trying, or also just leave the app store.
Slotting fees are just at contentious as Apple's 30% cut but the comparison is not apt as it would only work if Walmart were the only outlet available to any supplier to sell their goods. Walmart does not have a monopoly whereas Apple does when it comes to selling to Apple users. If Walmart were to demand a 30% cut for your product there are many other venues you can turn to up to and including opening your own store. This does not work in the company town of Appleton where only one retailer - Apple - is allowed to sell goods. Want to sell in Appleton? You'll have to go through Apple who gets to set the rules.
It's not a retail store. The App Store does not purchase, hold, and resell inventory from wholesalers. These are necessary functions to be considered retail.
Spotify isn’t giving Apple a cut of revenue per se, so much as it’s spending money on ads for a specific audience in a specific way.
I suppose Google too could make Android phones more attractive.