Yes, It Really Is Against US Law To Boycott Israel

'The Bureau is charged with administering and enforcing the Antiboycott Laws under the Export Administration Act. Those laws discourage, and in some circumstances, prohibit U.S. companies from furthering or supporting the boycott of Israel sponsored by the Arab League, and certain Moslem countries, including complying with certain requests for information designed to verify compliance with the boycott. Compliance with such requests may be prohibited by the Export Administration Regulations (EAR) and may be reportable to the Bureau.'
Anitboycott Compliance