If another marketplace takes a lower cut, allowing the seller to set a lower price while keeping margins, they can’t do so under this arrangement without taking a hit on Amazon sales.
The market should be encouraging finding the lowest-margin distribution path, not artificially propping up prices elsewhere to match Amazon’s margins.
But you care if it costs you $10 instead of $9. My point is that Amazon's rules means the price is higher for everyone, in order to cover Amazon's fees. Without them, if you are willing to buy from somewhere less convenient than amazon, you can get a better price.
That is not the only reason free market capitalists want competition. Competition should also promote a variety of choice, innovation, and dynamism in other services to consumers, not just low product prices.
Also, the only reason we want competition is because it should get lower prices for consumers. Competition isn't a good in and of itself, it's just a means to an end. So if more competition means higher prices for the same goods, then who needs it?
I don't quite get your second point: if the consumer is paying the same price, why do they care whether 100% goes to the supplier, or 99% or 0.01? If an item costs me 10USD, I don't care who get's that money, I just care that I don't get to keep it right