People will always find ways to get the new releases, everyone's answer is true to a point, some people swipe them, some find ways to hack it, regardless, it gets leaked all over torrent sites and the party starts.
Everything on the market place are on MS servers (obviously). now everything in the servers is kind of like a blank canvas. It has the data and the game and all that but its pretty much useless (for most people) until it is signed to your xbox and all that good stuff that your xbox does behind the scenes.
Everything on the market place can be accessed via HTTP and you can dowload it. However it will be in a .xcp file not signed or anything. After they found out how to download it they were able to extract it and get it to be playable on a jtag since a jtag can play somethign thats unsigned.
Or atleast thats what my understanding has come to on how halo reach got leaked. that maybe wrong.