Economic boycott of this sort of thing is, in principle, actually *protected* by the First Amendment; it’s part of freedom of association. It’s no different from say, me telling a friend of mine that I won’t have anything to do with him anymore until he ceases to patronize a restaurant that won’t serve Jews. I am free to say that, he is free to comply or ignore it.
That distinction — checking for physical or legal coercion — is absolutely key to evaluating whether something is a threat to the First Amendment (which IMO codifies freedom of thought, commnication and association).
It doesn’t mean that MoveOn’s action is any less contemptible, for the reasons explained here; it’s a confession that MoveOn and its ilk do not believe that they have a real chance in a fair ideological fight. After all, this same impulse is behind their attempt to resurrect the Fairness Doctrine, which actually IS a threat to Amendment 1.
But you have to be careful with this stuff; don’t cut off your nose to spite your face. Lmpw the difference between private, free action, and the censoring power of legal and physical coercion. Economic boycott is an instance of the first, an exercise of free association. That it is an ineffectual and revealing one, in this case, is just another reason to love freedom 
