Because the now they will never meet you in real life and there coward so they take advantage of the moment, but your on black ops dude by a headphone and yell at them to then shoot them. P.S. most of them are stupid 9 year olds
People are always braver when they can remain anonymous. Just as you don't know them, they don't know you so don't take it personally. Some people are just that way. It just shows they are immature and not very nice at heart.
Look man that's just how people are there's no changing it. And really if you are going to play Blac Ops than there will always going to be people like that find another game if it annoys you too much.