[GreedClarifies]

Who has more "power" in the arrangement seems to be a function of the business cycle. Employees, in tech (we are on hackernews), had far more power 1-2 years ago.

Why does there need to be a law here? Why don't employees say: "If you want a non-compete you have to pay me $X/hr more" or conversely if non-competes are common : "you can pay me $X/hr less if there is no non-compete".

That this doesn't happen, and that there isn't a wiley entrepreneur out there figuring out that "if everyone would take X less for non-compete job, I'll offer then X/2 and make bank!". Since this doesn't happen, this means that employees don't care much at all.