"Free" in the sense that they have the 1st Amendment right to do so. Not in the sense that they actually can, especially once Walmart gets to be a significant customer to them.

How is that any different than any business that has one large customer?

And if you're creating a valuable product at a good price, then you have leverage over Walmart. If you're selling a commodity, then be prepared to get squeezed. That's how it should work.

No, that's not how it should work at all.

Really? So if someone else charges a lower price for the same product, I should be protected from a loss of business?

That goes against our entire economic system. We'd still be driving shitty expensive cars if that were true.