A lot of people downloaded the game rather than getting a disc, so they certainly don't need a disc in while playing. It'd be weird to require it only for the subset of players who had a disc in the first place. Was it some sort of installation option, where if you have a disc you don't need to install the full game to your harddrive? That's the only thing I can think of. Some games allow either a full install to harddrive (then no longer needing the disc) or partially playing from disc (so you save some space on your harddrive). That was more common a few years back, though, when harddrive space was more limited and expensive.