If you buy a movie, you are buying the rights to private use of the movie, you aren't buying the copyright. You can sell a DVD movie to someone else and it's not illegal and doesn't subject you to copyright law.

If you buy a game that has a license key, then yeah, you are buying a license to the game even if it has physical media, but buying a physical copy of an Xbox game doesn't have a license key (well, more recently they do, the box contains a store key instead of a disc, but before that was common practice)