On the topic of ground employees not having to go through security....from what I can tell, it may be largely based on the individual airport itself and even the layout of the terminal.
At PHX where I work, all US and WN ground employees must go through security to get to work, juts like everyone else. However, a couple thousand feet away over in Terminal 2, employees can easily and legally bypass security all they want, primarily because that terminal has ports of entry to the sterile area that are in private areas.
When I used to work in Terminal 2, (the oldest and smallest terminal, mind you), I think I might have gone through security 1 or 2 times a month. Now I go through everyday at Terminal 4. Food for thought.
Im guessing PHL just has a layout that makes it easy to enter through other areas. This guy could have actually pulled it off if they hadnt traded bags in plain sight, but rather, take his bag down to the ramp, throw a gate-check tag on it and throw it in the bin with everything else.