I'm sure someone here has harder info on the letterboxing of commercials, but
I think part of the answer must be the annoying and increasingly frequent
insertion of commericals into the pre-film trailers in movie theaters.  I
suppose that makes the commercials look better--and really well-done ones
can be entertaining, but it is no less annoying for all that.
 
--Don Larsson, Mankato State U., MN