If we are talking about Zombie Squad, it has to do with the origions of the group. It started after watching zombie movies and asking "what if". It has become a metaphor, based on the theory that if you are ready to survive zombies, you can survive almost anything. I would encourage you to take a look at their faq, http://zombiehunters.org/faq.php.

If you start talking about all the heavy stuff, it scares people off. This is a lighter way of looking at it. Right now, zombies are the silly disaster of the day, 20 years ago it was mutants. And if you don't get the whole "zombies are a metaphor" part, deep breath, sit down, take things a little less seriously.