Because people do this waaay too often.
I see that many of you relate it first and foremost to flaming, but that's not quite what I meant. With flaming, you at least can understand the aim of the person.
This stamp, however, was inspired by situations when the person is actually trying to be friendly... or I don't even know. it's such a situation when you don't even know how to react and the best answer you can think of is "oh, okay
someone on the Internet..."