They didn't print an entire AR-15. They printed the lower, which is the pistol grip, trigger, etc. The barrel, bolt, etc were metal and were not made on the 3D printer.
I'm pretty sure a gun made entirely out of plastic would only work once, and would stand a good chance of injuring the shooter.
I bet you could use a 3D printer to print a working water gun, so I see your point about needing to ban 3D printers, in the name of security.