They can't add achievements to indie games because it would completely undermine the achievement system. You'd see countless shitty 80pt games where you press the start button and unlock the achievements. Having to add another layer to the peer review system to check for achievement validation would just make the process more annoying and arbitrary, the only reason your game should not pass is if there's some major bugs or interface issues.
The whole idea for indie games is so anyone can make a game without any major restrictions than an XBLA title would get. You can add your own achievements to the game, but if you want it to increase gamerscore, better start fishing for an XBLA contract.
