Yeah, I don't think you've ever set one of these off before. It's not nitroglycerin, but they can be very dangerous, especially in a "secure" environment:
Furthermore, what does it matter if it's in a "secure" environment. Does CO2 sublimate faster on one site of a security checkpoint? Does a PET bottle become a lethal weapon when X-rayed in your luggage?
It was a stupid prank. The guy should be fired. 6 years in prison is a ridiculous prospect.
6 years in prison is quite excessive, but the punishment should certainly be more than "Oh you're fired." This person intentionally caused a major public disturbance, and should be fined accordingly. Maybe probation.
There's "putting a flaming bag of poop on your neighbor's doorstep" and then there's "delaying flights out of LAX".