I figure that sometimes they're not actually trying to be sarcastic. They say something that is plain stupid or ignorant, something that clearly is not sarcasm, and when people barge in to berate them for their idoicy, they reply that they were being sarcastic as a defensive measure.
Yeah, that too. That kind of thing is also pretty spineless.
Yes, and if you simply disagree with something said (or overlook something someone said or misunderstand it) you're a "****ing moron" or whatever other insult implying idiocy.
And when you call them on it and comment on their utter lack of courtesy you're called ''too sensitive'' followed by vulgar name. Which is funny, considering they'll fly off the handle for the slightest reason.