Grocery stores are a low margin business. If you make selling shelf space illegal, they lose that revenue and will have to raise food prices to stay in business. This isn’t a good outcome. I also question if the shelves would even changes much. They will probably prioritize their high margin products, which doesn’t sound any better.

Where does the money to pay for shelf space come from if not the money we pay for food?

In theory, sure. In practice, the food makers aren’t going to lower their prices to the stores, they will just stop paying the shelf fees.

What’s the legal way to arrange things on a shelf?