That’s insanity. Under those terms, most suppliers would be at the whims of platform owners.
Congrats, you found one of the many problems with the concept of capitalism.
Microsoft asking Valve for a 50% cut? Sure. Google delisting a website because the owner criticized their CEO? Absolutely. Amazon telling you to sell at a loss or not sell at all? Why not.
Yes. That is how all that works. Because they are all private companies and it is voluntary to use their services.
If you think those services should be neutral, congrats, you're advocating for communism. I think communism is pretty cool and there should be a state-run online marketplace that is entirely non-profit. But you seem to think you should or could force companies to be that public entity. That's not only not realistic and not how the world has ever worked under capitalism, but it's just simply a bad idea. Look at the USPS for why you should not have a private entity perform a public service.
I'm really not. You want a neutral marketplace that just provides a service without a profit motive. That's what it comes down to. Because any service that requires a profit will naturally protect that profit, and that includes excluding suppliers that are actively sabotaging it.