Wait… But, he just cuts those off?? Law's powers have shown that he can cut stuff like that, doesn't matter if it's hard as seastone or diamond. Besides, he's not really "cutting" them in the sense we thinking of cutting. He's separating them. Let's think of Law's powers in this sense: when he creates a "Room", it's like he's creating his own dimension of sorts. We can gain information about these powers from looking at it's effects on living things. When cutting a limb off a person, they don't start bleeding or anything. Therefore, we can interpret that the limb and torso from said cut individual are still attached in some way, just not in a way we can see in the metaphysical universe. So, the chains are still separated, but not cut. They're still attached somehow, just not in a way we're shown. Kind of think of it as a small-scale wormhole or something.
Of course, this poses a question: what happens to that person who's limb was cut off, if they leave the "Room"? Bloody, or still connected?