really can't say i wouldn't do the same. if i was tired and DJing and some jackass threw a full can of beer at me, i would probably do a lot worse. justin beiber got hit by a bottle of water that was opened and had some sips taken out. used water bottle is a forgetload softer than an unopened can of beer. not to mention he's standing in front of a bunch of led lights, imagine if it had hit the lights, it could forget up all the electronics attached to that backboard. i'm not saying what he did was right, i'm just saying it's what i would do.
oh yeah, and if you're forgetting, he did make the music in the first place. DJs have done the same thing with other people's songs since discos and nightclubs existed.