It wasn't demonstrated, it was assumed. Mostly because you will need broad understanding of economics. I'll try to do my best to demonstrate it. Imagine a smaller economy where there is only 1 firm. They have complete power to set wages, you don't like it you don't work. (We've already established this would happen eventually if agreements like this were allowed because it is far easier to control wages than it is to create new and competing firms.) What happens when they decide not to pay a living wage? More importantly, what is to stop them from influencing laws such that what becomes fair practice is changed over time? Is it fair to allow a single entity the power over not just the economy but the power to define what is just? We have to be very careful in a capitalist society about confining the limits of what actors are allowed to do because quite literally money often equates to power and with time power defines law. I hope it goes without saying that enough bad laws over a long enough time span are trouble for everyone in society. Unregulated capitalism will always lead to issues for a society because it always concentrates wealth. (Those with the most information and the best decisions always come out more ahead than the rest.) Remember, capitalism isn't the best way to run an economy its just the least bad.
Also, please remember you can't look at a statement in a vacuum. "Why isn't it? Those agreements don't restrict or infringe on the freedom of others to form different agreements, or avoid forming them." Perhaps not technically, but realistically they absolutely do. If we had a million worlds with a trillion firms and easy travel between them then such actions would automatically correct, but we don't and thus people do not realistically have the option to turn them down. The whole point of capitalism is to reward and penalize decisions. It kind of breaks down (predictably) when one side has enough power to prevent the downside of a bad decision.
Remember, free speech doesn't mean you can run into a theater and yell fire.