There really IS a very forceful pro-zionist lobby and a lot of pro-zionist money sloshing around in the U.S. All this is post '67 ('73?): the watershed moment when U.S. middle east policy shifted to recognize the extreme usefulness of Israel as a proxy force and some American jews recognized the extreme usefulness of supporting U.S. foreign policy AND a way of playing the identity politics game that was cropping up.
The short answer to your question is that criticism of Israel is explicitly criticism of U.S. foreign policy in the Middle East -- because without U.S. support (enormous financial and military support), the Israeli regime could not do what it is doing and what it has been doing for the last fifty years.
It IS also the case, that pro Imperial Zionist propaganda has convinced everyone that being anti-Israel is the same as being anti-semitic.
Joanna