I assume if that were an option, it would be tactily endorsing mod chips, which are the only ways to play burned discs on a console. I think we can safely say the console makers and game publishers do not want to endorse mod chips.
Still, for those with a Live account, it'd be nice if they'd let us download demos. Say, in a Steam kind of way. That would definitely be an incentive for more people to sign up for Live, adding more value to the service.