Hey Joe, if I were elected POTUS, would I be equally "immune" from criticism because to do so would be homophobic? I haven't noticed folks being shy about criticizing Obama by the way (heck even his own party members dump on him from time to time). I guess they don't mind being called racists when they do so or something.
The idea usually is that criticism of Obama is only racist when it comes from the right-wing of the Republican party. Tea Partiers quite understandably get tired of the accusation, and so may express their frustration in unhelpful ways.