The only thing I can think of is, when they are new and unused they just sit there, but when you have burnt them you are constantly moving them about to get to the disk you are trying to find. Therefore theres more chance of scratching them.
Might not be the reason but its the only thing I can think of. Once burnt I always keep my disks in a case or a sleeve anyway to stop it getting scratched and to make it easier to find.
Seems like an odd comment for them to make though.