[EDIT: Duh. I misread the original post. Of course, the salary should be higher not lower to compensate for such a restriction--though I still suspect it would be small.]
Market price is determined (in a well-functioning market) by the intersection of supply and demand. So the salary could go other way: higher, if a non-compete makes it harder for the employer to find employees willing to sign the clause, lower, if result is fewer alternatives and/or more desperate employees interested in the position.
I'd actually argue the second: that noncompetes raise employees' switching costs, and reduce their alternatives, so that a state in which noncompetes are valid and widely applied would tend to have depressed labor rates.