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.
I checked Wikipedia and it had a pretty terse definition: the sale of goods and services to consumers. I have to agree with GP that another definition seems to have been selected for the sake of excluding the App Store.
Whether or not one thinks they deserve a 30% cut, Apple definitely works to make sure the App Store can at least deliver the application to the hardware and their interfaces ultimately bring Spotify's interface to the user. That fairly reasonably fits in with "the sale of goods and services to consumers". In-app purchases are more arguable (keep in mind it necessarily uses Apple Pay) but this thinking that the App Store is not comparable to retail seems to be a little narrow-minded.
Apple is to Spotify as is the construction company which made the building Walmart is using to Walmart.
I have not heard of construction companies claiming entitlement to a cut of Walmart's turnover.