It's cause they're collectibles. They know we duel, but it says not to, probably because they don't want everyone sending in their broken lightsabers for repair, when it was their fault in the first place. They also don't want any stupid lawsuits, just in case something weird happened, such as a saber being duelled with so much, that the blade came out and hit some kid in the eye. They could say, "BUT IT SAID PERMANENTLY ATTACTHED TO THE BLADE!" I don't know how far that would fly, but you probably get what I mean.