We see the same thing in Oslo, Norway where the local government is trying to reduce car use in the city. The main concern is from people with disabilities who fear for their disability parking. I believe it is because the main mobility device they have been provided is a car, so they have been used to moving about with a car. That they can use electric mobility scooters, electric trikes and a number of other mobility devices and also public transportation like everyone else doesn't seem to cross any ones mind.

I saw these all the time when I lived in Copenhagen as well. NYC needs to make its subway be more accessible for this to be a proper solution, though. In Copenhagen it's easy to take your scooter on the Metro or S-Train for longer distances, and then drive in the bike-lane network for shorter distances. But many NYC subway stations are ancient and don't have accessible entrances, so you can't get a mobility scooter from the street onto the train and back up to the street.

I’ve heard from a dwarven friend in SF that the SFBART elevators are also frequently used as toilets. Is this also true of the existing subway elevators?

It might work if NYC got better at hauling garbage, clearing snow, and ticketing people for blocking sidewalks in other ways (tables, signs, delivery trucks, etc.).