Actually its a misconception to say that they're not moving. This bug when you use bare fists, barret, ruger and saw (as far as I know).
Do you know that the bots will still move, but they only move vertically?
Try this:
Get a map like inf_zomcano (its one of the maps in Toumaz's Zombie CTF server). Even if you carry a ruger, barret etc, the Zombies WILL still charge down towards you from top to bottom for some reason. When you go above zombies, they will also fly upwards to try and stop you, but they never move horizontally.