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.

I'm pretty sure retailers pay wholesalers, not the other way around.

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.