Actually, I don't understand the complaint at all. I don't think I've come across a single game that doesn't keep your single player and multiplayer progressions separate, and even though it might just be because I don't get to play much lately, I think the reason the developers did this so was to lengthen the duration of the game.
But then again, I suppose it makes sense if the things you have to unlock on one mode are the same as the ones that you have to unlock in the other, in which case, it's not gameplay lengthening, it's just pointless repetition
But then again, I suppose it makes sense if the things you have to unlock on one mode are the same as the ones that you have to unlock in the other, in which case, it's not gameplay lengthening, it's just pointless repetition