Those in support of the bill say it doesn't actually infringe on free speech or punish political thought. The key is the bill specifically bans people from joining anti-Israeli boycotts, which are led by foreign powers. If a person chooses to boycott Israel out of their own political beliefs, they're still free to do so.
The bill, strongly supported by the pro-Israel lobby AIPAC and the evangelical group Christians United for Israel, would expand existing legislation, adopted in 1977 in order to counter the Arab League’s boycott on Israel, to include also anti-Israel boycotts initiated by international bodies such as the United Nations or the European Union. The bill prohibits American businesses and individuals from adhering to boycotts not sanctioned by the U.S. government and includes hefty penalties to those who follow the boycott.
Critics of the bill say that argument is an arbitrary distinction. The effect of the bill is to criminalize a specific political belief. The ACLU says the First Amendment covers boycotts to achieve political goals. It places unconstitutional limits on free speech. It's specifically targeting people who support international boycotts against the Israeli government and only them.
This bill is not an anti-discrimination bill, it's anti-free speech legislation—and unconstitutional. No matter how nasty or offensive the boycott is, the government cannot punish a U.S. citizen for their political beliefs.
Under the bill, however, only a person whose lack of business ties to Israel is politically motivated would be subject to fines and imprisonment — even though there are many others who engage in the same behavior. In short the bill would punish business and individuals, based solely on their point of view. Such a penalty is in direct violation of the First Amendment.