I haven't spent much time in NYC but I had never seen people go through the doors as passengers until then. It seemed to be frequent/common too. I've since seen this once in another city and it was pretty clear that it was out-of-the-ordinary when it happened.

Here in Japan it's perfectly alright, so it varies from place to place. I know the one time I went to New York I saw a guy passing through into other cars, but he didn't seem to be the type to care about the law.