Mr Mxyzptlk said:
That's because you assume STFU to mean an insult. It could mean anything.
The accepted (usual) use isn't an insult - it just means 'shut the f*** up', and the context is important - a little gutteral, but banter nonetheless as in
Take your medicine and STFU now and again instead of continually shooting yourself in the foot (any toes left, by the way) and we'll all get along fine.
There are other meanings, of course...
Stop ThinkGeek From Understanding
Stinky Telnet Fell Under
Somebody Thinks Fitzgerald's Ugly
Slashdot Trolls Feign Understanding
Star Trek Federation University
Some Teamsters Form Unions
Salty Taste From Uruguay
Strangely Talkative Funky Underpants
Something Taken From Uranus
Stupid Tooextracool's Farting Urethra
... the list is endless.
If I'd said 'STFU you no-nothing annoying idiot, you're just being a whiny dumbass as usual' to anyone THEN it would have been contained within an insult, but of course being a staff member I wouldn't dream of saying such a thing.