It' called copywrite laws.
PS, did you even read that guide? Yes, you can copy your games to the hard drive on the Xbox 360 but just as i said above you still need the original disc in your tray to play them.
"Note The game disc must be in the disc drive, even though you have installed the game on your hard drive. The console will automatically play the game from your hard drive, but the disc has to be present."
They did this because if they did not then you would have no need to buy games, you could just barrow a friends and install it.