This is incorrect. Although, they do need to get permission from the original publisher, they are porting the games. How can you tell? Just have a look at the filename of the file downloaded. It has x64 attached to it.
Behind the scenes, Microsoft has built a full Xbox 360 emulator for its Xbox One console. "Xbox One Backward Compatibility is an Xbox 360 emulator that runs on Xbox One and is used to play Xbox 360 games," says a Microsoft spokesperson in a statement to The Verge.
http://www.theverge.com/2015/6/15/8785955/microsoft-xbox-one...
the package they are sending when you insert the disc is a container that contains the xbox emulator, any media assets, the game and compatibility stuff that is needed.
For example running original Xbox games on Xbox 360 also required a download of a binary which was an emulation / compatibility wrapper around the binary on the DVD that shimmed out and hotpatched code so the games worked properly.