What only one person has brought up in this thread is that the announcement of backwards compatibility specifically mentioned that developers need only to tell Microsoft that they want their 360 games on the One, and it'll happen. No effort whatsoever on the part of developers. So unless Microsoft plans on shouldering the full burden of remaking every title for which they're given the green light, then something has been changed deeper in the One's structure to facilitate the cross-compatibility of every 360 game ever made.
In other words, Microsoft shouldn't need the source code of any game that they don't already own. Furthermore, it sounds like they've already done all the of they work that they need to do on this feature for now. They'll probably encounter bugs when it rolls out live, but they won't have to start from scratch.
As for the current list of games being only titles that Microsoft had a hand in, it's safe to say that Microsoft doesn't need Microsoft's permission to allow their games' backwards compatibility.
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Still, I DO wish they would allow compatibility to work right off the discs. So many good games will never be part of the program due to licensing issues that currently disallow the sale of digital copies on the systems for which the games were made.
I'm looking at you, Deadpool Activision...