I think I saw a quote (or heard it somewhere). I dont remember the exact quote but it basically said you use prepping for zombies as a motivation to prepare for a real emergency.
I would have to agree with this...I know (or do I?) that zombies are not going to take over the world, but it is fun to use it as a motivation to prepare my family for an earthquake or other emergency.
I think when it comes to DHS, the zombies are there to represent John and Jill America. The public thinks its fun when the DHS trainees are killing "zombies". If the trainees would have been "shooting" John and Jill's, that may have been not so cute and fun.