Do you think this has something to do with the fact that turnstile jumping has been effectively legalised (in the sense of not being prosecuted) in NY?

Question for you: can you quantify, what fraction of crime and other forms of sociopathy in the NY public transport system you estimate to be committed by passengers who paid their fare? (My estimation: less than 1 percent.)

I don't think it's reasonable to assume that a simple metal gate alone can completely solve complex social dysfunction, a simple metal gate can however help, and, when we refer to turnstile access being desirable, we implicitly assume that we can reasonably expect turnstile use being adhered to, and violations punished with at least moderately high probability.