That's called LETTERBOX mode. Check your tv to see what presentation mode you have it in.
A widescreen movie is not cut off on the top and bottom. Your HD tv is 16x9 format, which is a compromise between 4:3 televsion of old and 21:9 cinemascope movie format. Many movies are now filmed in 16:9 format so that the full hd screen is filled with an image but the vast majority of movies were filmed in a wider format.
It can be a little disappointing on a 50 inch screen but looks great on my 8 foot screen.
