One interesting but often-overlooked aspect of the debate over the proposed Israel Anti-Boycott act is how it merges two very distinct cultural arenas where Israel boycott politics play out. In the United States, the BDS movement is (for the time being) largely about individuals or small organizations making their own decisions of conscience. Now to be clear, one can make a bad decision "of conscience" -- a baker who refuses, as a matter "of conscience", to bake a cake for a gay couple would be a great example. So I don't mean that as an apologia.
What I mean is that Israel-boycotters, in the US, are making a decision that by and large is not tied to state-coercive power. That may well be less the result of any principled limitations and more because prospective boycotters do not have the political power to enlist state-coercive power, but it is a descriptive quality that is fairly noted. Likewise, the line between "boycotts" or "conscience" and "discrimination" is a thin one (as the anti-gay cake-bakers demonstrate). But there is at least a plausible claim that liberty allows one to expressively refuse to associate with those one hates -- even if one's hatred is nothing but hatred, even if the expression is naught but prejudice.
Internationally, things are different. "Boycotting" Israel, in many parts of the world, is an exercise of governmental-diktat, a feature of state coercion and an impediment to human liberty. Iran, for example, just handed down a lifetime ban against two soccer players who (while on the roster of a Greek team) participated in a match against an Israeli squad. Another Iranian journalist just was granted asylum in Israel after Iran threatened to prosecute her for "spying" due to her publication of articles in an Israeli newspaper. A Bangladeshi politician was charged with sedition after meeting with an Israeli official in neighboring India; Bangladesh has no diplomatic relations with Israel and citizens are forbidden from traveling there. The original law that the Israel Anti-Boycott Act is modifying was addressed to the latter problem -- states which were seeking to mandate a boycott against Israel as an exercise of coercive state power (in my post, I discuss this as the secondary boycott problem).
In these contexts, the movement to boycott Israel takes on a (more) unambiguously illiberal bent. It's primary function is as an annex to state repression. To speak of the movement to boycott Israel as an international movement without considering these contexts is badly incomplete. To be sure, one can, I think, defend the right of individuals to boycott whomever they like as a form of free expression (subject, of course, to the difficult parsing that requires vis-a-vis anti-discrimination requirements -- a parsing that itself has not been addressed with requisite seriousness). But a discourse about boycotts that takes "free speech" as its foundation but which does not account for these cases is fundamentally dishonest. The fact that the movement to boycott Israel has, in its original (and longest-standing) manifestation taken such a clearly oppressive form is something that should be dwelled upon.