On bds it works. So there could be some hacky way to enable it. However when you do it, you can earn what achievement you want. So that's the reason why it is disabled.
There is a field called hasAchievementsDisabled in StartGamePacket. I have not tested it but it could work if you set it to false.
xBox achievements are tied to your xBox live account. You can only get them in single-player mode. There's a reason for this..something about them being tied to rewards in your Microsoft account.