Why Apologize?
Because being polite and showing that one can show others respect is always a nice skill to have regardless of the medium used to communicate.
One of the big problems of the web is that many tend to see the people they are interacting with as somehow less then 'real' and thus feel free to behave like ignorant oafs.
Those folks tend to bring the entire space they interact down to their level by starting argumants, randomly insultign folks, and being the sort of person you'd call the management on if you encountered them in the mall or a theater.
With his apology, AA is showing a touch of civility and indicating that he felt he has overstepped the boundries of politelness.
Myriads