And that's their choice.

If I go to the trouble of building a store (bricks n mortar or online) then I'm going to sell what I want to sell.

I built the fruit store, NO, I don't want to sell live Aardvarks from my store, and it's preposterous that you think you can force me to do that.

It's more like I will sell the apples and oranges of all the farms except for that one farmer who dared to try to open up an apple store. At some point, if you are the only apple and orange store in the area, the government may decide you are abusing your power as the biggest such store and are acting anti-competitively

Surely it only makes sense that I prioritize selling MY apples in MY store, and no somebody elses.

I built a store to sell my stuff and make money, after all.

Sure, if your store mainly sells your apples do whatever you want. But once you open everyone else to sell their apples at your store for a cut, and everyone starts to rely on it as the only apple marketplace, screwing over the apple farmer next door who tried to open his own apple store is questionable