that's probably because the demo time period has passed
the demo stops working as soon as the full version came out, or so I would imagine.
In the 2nd demo stage, I tried staying in the vents because we cleared everything out already... next thing I know I'm getting mauled by a smoker that randomly spawned in there
my friends were all like... fuck, why did you stay in there when we all ran out already?
random spawning of hordes are AWESOME, my friends finally killed tank outside the train area, next thing you know we're getting zombie rushed ...and we died.