In a good market, only more desperate candidates will go with non-compete positions. So lower salary argument makes sense
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.