The players have every right to defend themselves, Artest might have been out of line for jumping into the stands but in the heat of the moment who can blame him. The verbal abuse always goes on, but when you start throwing things IMO you're a coward. If the fan was that upset be a man about it and confront him 1 on 1. Although by the look on that fans face Artest would've definately opened a can on him.
O'neal had every right also, to defend his teamate (I believe that was the same fan that was swinging at Artest from behind while he was being held in the stands). The league should review the tape and ban all the idiot fans involved.