It’s not just left-wingers who are being detained. In July, Meyer Koplow, the chairman of Brandeis University’s board and a pro-Israel philanthropist, said he was aggressively interrogated at the airport after security officials discovered a brochure in his luggage called “This Week in Palestine.” (He had gone on an educational tour of the West Bank during his trip.)

Israelis have good reason to see the B.D.S. campaign as a thinly veiled form of bigotry. Boycotts of Jewish businesses have a particularly foul pedigree in Nazi Germany. And the same activists who obsessively seek to punish and isolate Israel for its occupation of the West Bank rarely if ever display the same passion for protesting against China for its occupation of Tibet, or Russia for its occupation of Crimea and eastern Ukraine.

It’s also true that Students for Justice in Palestine has received funding and other assistance from a group called American Muslims for Palestine, some of whose leaders have links to groups flagged by the U.S. Department of the Treasury for their ties to the terrorist group Hamas. The group seeks to end Israel’s “occupation and colonization of all Arab lands” along with “promoting the rights of Palestinian refugees to return to their homes” — language that has long been code for dismantling the Jewish state.

Israel, like all countries, has a right to protect its borders and to determine who is allowed in and out. But Israel is also a state that prides itself on being a liberal democracy — a fact that goes far to explain the longstanding support for Israel among American Jews and non-Jews alike. If liberalism is about anything, it’s about deep tolerance for opinions we find foolish, dangerous and antithetical to our own.

The case for such liberalism today is both pragmatic and principled. In practice, expelling visitors who favor the B.D.S. movement does little if anything to make Israel more secure. But it powerfully reinforces the prejudice of those visitors (along with their supporters) that Israel is a discriminatory police state. If the Israeli government takes umbrage — and rightly so — when Israeli academics or institutions are boycotted by foreign universities, the least it could do is not replicate their illiberal behavior.

Detaining people like Ms. Alqasem also does little to stem a worrying trend among young American Jews, who are increasingly alienated from Israel because of its hard-line policies.

But the principled case against this paranoid policy is even more important. Theodor Herzl’s vision for a Jewish state was not the “ethnostate” of Bannonite fantasy but an open, pluralistic society. Liberal societies thrive not by expelling critics but by tolerating and even assimilating them — and therefore defanging them.