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'm always a big fan of if you approach somebody politely about something and you're not a nudge - you're just pretty honest and simple, my kind of philosophy is that I'm not afraid of 'no,' and that's way different than 'I won't take no for an answer.'