One key thing that a lot of people seem to be missing in this conversation: The turnstiles rotate in two directions, inwards and outwards. They always freely

One key thing that a lot of people seem to be missing in this conversation: The turnstiles rotate in two directions, inwards and outwards. They always freely rotate outwards, so every turnstile can always be used to exit the system. They are always locked against inwards rotation, except for when you swipe your Metrocard, then they perform one partial rotation to admit you. The linked photo shows someone who is being denied entry into the system, i.e. payments cannot be made and thus that one partial inwards rotation cannot happen. Outwards rotation, however, remains unaffected; if they were on the other side of the turnstile, they wouldn't be trapped inside; they'd still be able to exit through the turnstile.