Not necessarily. Do you need someone to tell you how to behave in public?

Of course not.

And, funny thing, I'll bet most of the poison pen posters don't think they're
behaving badly (by their own standards, at least). One of that crowd recently
made some very childish remarks about the sexuality of some other posters, then
tried to deny he'd done so, and in the end said "well, you would have thought
it was funny if you had a properly developed sense of humor."
Do I suspect this is the way a guy like that
behaves when not on-line? Absolutely.

in The Real World(tm), life generally goes
quite a bit more smoothly when we are courteous to others.

It's dangerous to presume that others have
any interest in life going smoothly. Some kids just love to break things.