Exactly, I think what it really boils down to is that one of the number one things that attracts some gamers to consoles rather than PCs is that when you buy a console, you know that you'll be able to play every game in that consoles lifetime without having to upgrade or worry about specs. If MS allowed devs to put in an installation requirement after selling 360s that had no hard drive, they would essentially be having system requirements for console games, which could potentially turn a lot of people away from the xbox.
Personally, I am a PC gamer and have no problem with system requirements, but I can understand why gamers who can't afford to upgrade their computer every few years would be attracted to consoles.
This is truth. The market for consoles is borne by assurance. You can't violate that without risking turning consoles into something that isn't a console.