I will concede that point, but with a caveat:
Walmart almost exclusively deals with major manufacturers who have the ability to sell in high volume and command large market shares - thus, they have a pretty good amount of leverage in any discussion. But when Walmart comes into a town and muscles out all the competition, you are still going to at least listen to what they have to say, and capitulate somewhat. It's not a total monopoly, but it's a lot of bargaining power on both sides of that table.
Amazon will let any mom&pop sell on their storefront. If you are a small retailer, you either play by Amazon's rules or gtfo.
So, I don't think Amazon has a whole lot more sway over the bigger companies than Walmart does - but Amazon lets smaller folks come play to, and yeah, they certainly have a lot more influence over the smaller guys than they really should.