There is no justification for smuggling weapons aboard. The Journos are not trying to provide a safety service, they are merely trying to make a quick buck out of selling a scare story. If they were interested in our safety, they'd conduct their research legitimately or use fake weapons.
If they tried to storm the flight deck, would your arguement "they're only journos testing security" stil hold water? What if they sabotaged aircraft (testing engineers etc)? Or having breached the flightdeck, interfered with the controls to "test the pilots abilities to recover a situation"? Where do you draw the line?
There are licensed and controlled groups who test security arrangements. Let them get on with the job, and stop defending self-interested, panic inducing and dangerous journalists.