Retail stores have a finite and very small amount of shelf space. Publishers pay a lot for a significant percentage of that shelf space devoted to their product.

The App Store has effectively infinite space. There are so many products and search is so broken that it's extremely difficult for users to discover your product.

Rather than comparing the experience to brick and mortar stores a more apt comparison would be to throw your products in a landfill, junkyard, or extremely large flea market and expect your users to find them. Would you pay high commissions to a junkyard for tossing your product in a pile "somewhere in the back"?

No not really. Can you imagine gamestop demanding a cut of dlc purchases because a physical game was originally purchased in their store?

These are separate concepts.