You don't pay a fare to exit the New York City Transit system, so nothing will prevent you from leaving during a fire; control system failure or otherwise. There are also emergency exit doors (that many people open to exit more quickly in normal operation of the subway). One may argue that having to go through a turnstile at all is an evacuation disaster, but presumably the designers thought about this and the exit doors are adequate in an emergency.

This is different than systems like London and Tokyo, where you pay for the distance you traveled, and a station evacuation mandates opening the fare controls to let people exit more quickly. For better or for worse, that option has never existed in New York.

Usually here are gates with a fire alarm next to them so you bypass them when needed.