How many times have we seen this: a performer makes his name on basis of his ethnicity and once he reaches fame forgets where he comes from? Add Jerry Seinfeld to a long list of such snobs. Did this guy even look at the calendar? I mean, even if you don't have Jewish origins the fact that a person is named Tamar Cohen is not answering your phone-calls on Saturday should give you some sort of clue! Is he an idiot?
The last was not a rhetorical question. Seinfeld is not an idiot, you don't become successful in the entertainment industry by being one. Something much more serious is at play here. It is the fact that the modern-day celebrities got so used to the God-like treatment they are being given by the media, that they all of a sudden think they have become gods. Thus, to them it is unthinkable that a person should have his principles and customs that transcede the celebritie's needs which are to be served immediately, no matter what. In time, the superiority complex grows so high it cuts one off his origins completely. Hence, Sienfeld's behaviour in this incident should not be too unusual because he himself is Jewish.