Other mistake: When the zombies come out of the river and start marching, look closely. The first zombies to come out wear wet clothes, but the ones in the back are dry.
Continuity mistake: When the bikers ride through the zombie infested town for the first time watch when Big Daddy is shot at. The windows behind him are shattered by the bullets but a few seconds later are fully intact. This may be hard to see as it is dark but they are back, easiest way to tell is by the reflection from across the street. (00:08:50)
Revealing mistake: At the beginning of the movie, when the guy is bitten in the wrist, if you watch and listen closely when he shoots himself, the blood comes out of his head before he actually shoots.
Continuity mistake: When Kaufman sees Big Daddy, he shoots at him, particularly in his right shoulder. When Big Daddy turns to look at Riley at the end, the bullet holes are gone.
Visible crew/equipment: When Dead Reckoning (the massive truck) is starting to be surrounded by zombies there is an extra spotlight mounted on top of it. It wasn't there in any of the earlier scenes. The shot where it's visible has Dead Reckoning on the right, eight or nine zombies on the left all walking by, and a few powerlines visible in the background. It looks like they just put it on top to give some extra light on the zombies.
Deliberate mistake: When Dead Reckoning is firing the missiles at the end (towards the crowd that's being eaten by zombies), you can tell that they repeat the same shot of the missile firing three separate times, even though it's supposed to be three different missiles. (It's the shot of the exhaust coming out of the back of the launcher.) They try to mask it by cutting a little earlier or later each time... but it's obvious.
Continuity mistake: During the scene at the club where the zombies are used as entertainment, someone sprays the checkered shirt zombie with red paint around his mouth. However, in the next shot from a distance, the spray paint is gone.
Answer: Riley feels that they have just the right to be there as the living do. Besides, those zombies survived all the missles and are coming to the city while Riley and the gang are leaving. So why waste the ammunition?
Jason Riley