I know about the duo-operators in MTA. Before I came to NYC I didn't understand it either.
After I moved here and saw how violent and hostile people are towards the train, I actually came around on the duo-operations. If MTA doesn't do that it's likely many many people will be crushed by the doors, and you'd get a lot of broken trains.
I never saw as many riders actively breaking the train doors in eg. Japan/HK/DC/Seattle/SF subway.