It sure seems wrong to me too. And as I noted, we always just take one. It feels like the right thing to do.
On the basis of Enid's queries I did some more reading up on it. You can get more info here. It appears that the commission in charge of the rules and regulations (The Florida Fishing and Wildlife Commission) have ok'd the practice - although they also acknowledge that it does leave the crab with less (to nil) defences when their claws are gone. They also note that the claw regrows back quicker the more that the crab can eat and grow strong - so with two claws they have more chance to get themselves food. So overall, morally, I am happier with our decision to leave this sustainable resource to rejuvenate with the best chances possible. And hopefully the more we have next season and each one onwards!!