Storm doors are required to be locked on 67 foot cars and up due to the swing of the cars when navigating tight corners and switches, and they are. 46, 68, and 68A doors are locked, as are the storm doors on the BMT Standards when they are used for Museum services, but from my understanding can be opened by the crew in an emergency. There's no rule requiring their locking on shorter cars, and I think until recently, it was not illegal to move between cars that way (hence the installation of pantograph gates on the 40 Slants. Hell, back 40 years ago before air conditioning, storm doors were clipped open to improve airflow in the cars.
The only reason I see for this rule (it being illegal to pass between cars using the storm doors in the absence of an emergency) being implemented in our time is the lawsuit happy nature of our society. Back in the day, if you f**ked up and injured yourself or died, it was understood that you were at fault. Nowadays, every idiot around the corner is trying to make a quick buck, and will sue anyone and anything for whatever they can get.