Question about Intense D

[potato]

well in most of the NBA games you see the big guys back to basket, slowly using his back to backoff the defender backwards in order to get to the basket, while most of the time the defender has to use his upper body to prevent the big man from getting closer, why would this be allowed, won't it be called an offensive foul for displacing the defender?

and if it's a defender pressuring a ball handler, can they get so close that the chest is in contact with the handler's body, it may not displace the handler but severly obstruct the handler's movement, or simply make the handler loose his balance. i'm asking because i'm annoyed when a guy starts closing in on 3 point line and presses his body on me, of course i can easily get away but when he 1st does it it's annoying.