The law doesn't prohibit monopoly by itself. Monopolization is only prohibited if it restrains trade, or if the monopoly position was improperly gained. If Amazon attains monopoly position through superior products, innovation, or business acumen, it is very much legal in the US[1].
I think it's hard to argue that Amazon undercutting the participants in its marketplace is restraining the trade: the complaint here is, as I understand it, that through better knowledge of the market, and better integrated and more efficient platform, it is able to offer same or better products at lower prices. I can't see how it restrains the trade, according to how FTC understands it. It would only be illegal if Amazon did sold these products below their own costs, and then planned to recoup the losses by raising the price after the competition is gone. I haven't seen any evidence that this is what's going on.
[1] - https://www.ftc.gov/tips-advice/competition-guidance/guide-a...
Whether these could be sufficiently proved is a whole other matter.
If you don't trade on Amazon's platform, you're not affected by any of these. You might as well complain about Safeway's (or whatever grocery chain operates in your area) anti-competitive practices, because Safeway will also do product tying via membership card, rewards and coupons, deny you facilities to put your products on their shelves, and won't pay carrying fees for its own store brand products.
Sure, it might be much harder for you to compete with Amazon if you can't use its platform, but then the argument is that the Amazon is too competitive, not anti-competitive, and that is in fact legal (and a boon for customers).
you can't prove this