They are warning people who are escaping that those doors do not lead to an exit. When you take a fire exit it should be made clear, by fire exit signs, where you are supposed to go, and which ways you should not go.
You see the same thing in buildings; go down the emergency stairs and each floor you pass should have a sign on the back saying its not a fire exit until you reach the ground floor when you'll see a door with an exit sign.
It might seem obvious, but when there is an emergency, amybe thick smoke obscuring views and people are frightened, its easy for them to be disorientated.