Corporations can artificially create a monopsony, just like they can artificially create a monopoly. The music industry, for example, relies on making its talent pool—who are in high demand by the customers—feel like they are in lower demand compared to the record labels themselves, so the industry will be in a better bargaining position.

Sure, once the market is as efficient as it can be, if there's still a monopsony, that's tough for the employee. But if the monopsony can be annihilated without affecting its constituent employers and employees, all the better for all involved.

ADBOC[1]. Your statement is technically true, but I disagree with the subtext.

Factually, workers with no marketable skills don't have them because they didn't acquire them as students. Factually, it's not Walmart's fault that its workers have no marketable skills and thus no alternatives. However, those two facts don't imply that it is just (or societally optimal) for us to do nothing about the problem.

Workers are being paid below market wages because their switching costs are too high. This is both inefficient (market prices are efficient prices) and unfair (the very poorest workers are being paid even less because they are poor). Therefore, it is right and good that we should pass laws to help improve that situation and return wages to their market values.

Now, there's the separate question of what the best laws to do that actually are. It seems unlikely to me that allowing employees to sue for something as nebulous as "infringement on free speech" is a good idea. But I hope I've made a strong case for why some legal intervention in this situation is reasonable.

[1] http://lesswrong.com/lw/4h/when_truth_isnt_enough/