Under the Digital Millennium Copyright Act (DMCA) of 1998, circumventing copy protection is illegal, even if you own the original disc. The Xbox uses a copy protection system called "Xbox Security Sector" that is encrypted. Breaking that encryption to create or download an ISO violates Title 17, Chapter 12 of the U.S. Code.
In most jurisdictions, downloading a ROM for a game you do not own is considered copyright infringement. The logic that "I own the disc, so I can download the digital version" is legally shaky; the safer legal route is to create your own backup from the disc you physically own. Xbox Roms Downloads