Not sure how much force the doors take on NYC subway, but other trains I've ridden on can be opened if you pull hard enough, but also not so easily that people don't risk slipping out by accident.

I've been on the train a couple of times when someone let their body rest right against the door and the momentum with their weight pushing sideways caused the doors to start opening while we were moving. so I think it's a balance of safety from both perspectives.