Is it illegal to carry a "closed" flask with alcohol on the NYC Subway?

I was going to a birthday party and decided to take my flask with me. I was on the train and I took the flask out of my pocket to make sure it was tighten, the police officer saw me and gave me a citation for an "Open Container." The container was not open, if it was, Vodka would have waste all over the place nor was I drinking it, I am not stupid.