A report from Spiegel Online has it all, including the following detail:
In Bremen in March, the party [the Left Party] refused to join a multi-party appeal against a further call to boycott Israeli products. The party allowed that the call was reminiscent of Nazi campaigns against patronizing Jewish shops in the 1930s, but said that boycotts against Israel were not anti-Semitic.
That would be, of course, because anti-Israel boycotts aren't motivated by the Jewishness of Israel but rather by other things - things of a kind for which one doesn't demand boycotts of non-Jewish countries. Now, if you can't follow the impeccable logic of this, you must be a Zionist.