It cannot be antisemitic. Ben & Jerry's boycott had to do with the state of Israel's aggression toward Palestinians. Antisemitism is to be against Jews because they are Jewish. Ben Cohen and Jerry Greenfield, the Jewish-American founders of Ben & Jerry's tweeted, "We continue to believe it is inconsistent with Ben & Jerry’s values for our ice cream to be sold in the Occupied Palestinian Territory." Nothing antisemitic about that.
Update on this story: Ben & Jerry’s Sues Parent Unilever to Freeze Sale of Israeli Business.The last sentence in the article makes sense: "No longer involved in Ben & Jerry’s operations, they [Ben Cohen and Jerry Greenfield] wrote in the New York Times last July that they supported Israel but opposed its 'illegal occupation' of the West Bank." It's like you can love someone, but want nothing to do with their shameful activity. And sometimes it actually takes a loved one to get the message through.