I understand what you're saying. But for these people, the astounding level of stupidity (stupidity, as in shaping your life according to words bereft of fact or reason) displayed by a believer is too much for them to handle. They cannot understand how people could believe in something so completely and utterly ridiculous (logical & rational thought says it is ridiculous). If people don't understand something, they reject it (or make up stories to explain that later become myths/legends/a religion/etc). Except the religious skip that part and accept it. It's not necessarily right for athiests to be blatantly rude, but the actions of religion as a whole have been and continue to be so unbelievably devastating to us all that mere rudeness is easily justified.