Because interrogations are intended to coerce confessions, interrogators feel themselves justified in using their coercive means. Consistency regarding the technique is not important; inducing anxiety and fear is the point.
I really believe that you grow up a certain way in New York. There's a New York morality, a sense of loyalty. You know how to win and lose. There's a thousand kids outside, you know who to push and who not to push. There's a sixth sense you develop just because it's New York.