Unless someone is splashing gasoline around, fire will spread very slowly in an open-topped plywood maze. And heat/fumes/smoke will head skyward, not accumulate as they would for an indoor fire.

As an organizer, I'd be much more concerned about medical emergencies and inter-personal crime in the maze.

I don't know anything about mazers (or fire safety), but, assuming flammable materials like wood, how do they extinguish the fire before it takes over the whole structure?

I guess "professional" mazes may have sprinkler systems. Still, it seems that even in buildings that do, a lot of value is put into maintaining escape routes.