Roxane Gay did a great post about etiquette and rejection over at the PANK blog. It reiterates the ongoing question of what makes a professional writer. Back when I helped edit Flesh & Blood years and years ago, I don't remember getting that many angry replies to the rejection letters we sent out, if any. Maybe it's a genre thing. Maybe genre writers are more respectful (timid?) than literary writers who, oftentimes, have their heads stuck so very far up their asses. Then again, some genre writers have their heads in the same very places too, so that can't be it either.
The last time I encountered a few rude replies (nothing near what Roxane has had to deal with) it was after sending out responses for the anthology:
Nobody likes to be rejected. We don't submit our stories to magazines hoping that they will be rejected. But at the same time, we can't expect our stories to be accepted to every magazine that we submit them to, either. Sometimes our stories won't be accepted at all. That's just the way it is.
If you're one of these writers who can't deal with rejection and fire off a nasty reply to whichever editor just rejected you (and let's face it: with the speed and accessibility that the Internet grants, clicking that REPLY button can sometimes be awfully tempting), then maybe you should ask yourself whether you should really be a writer in the first place.