There's never a justificiation for throwing things. We've seen a Man City player cut by a missile at Old Trafford last week and a steward knocked out at Eastlands the week before. I think Fizoxy makes a good point point about fans dishing it out but not being able to take it. Some players and managers are subjected to really nasty stuff from thousands, yet there is outrage if they react in even the tamest way. The Adebayor celebration towards the Arsenal fans was out of order, but they'd been screaming bile at him for the entire match, only for carnage to erupt when he got his own back. There's nothing wrong with banter between fans and players, but within certain boundaries. It just needs everyone to think about their actions, not difficult.
By the way, Fizoxy, the next time you hear the mystery racist, do humanity a favour and let the old bill know.