Oysters, like other bivalves, do not have any sort of central nervous system. Their nervous systems are actually a great deal more simple than the nervous systems of motile bivalves like scallops. The practical implication of this is that there's no agent in an oyster. There's no pain-feeler. read more
Using oysters to make pearls or to eat, then, is no more abuse than picking fruit off a tree is abuse. There has to be a someone to abuse for an action to be abuse and there's no someone in this situation. read more
Coral, to seaweeds, to fish species, to otters rely on oysters. Wiping out their ability to interrelate with oysters (even if it just means eating them) hurts them. Thus, buying pearls (at the end of the day) very well could be cruelty to animals, but on a scale beyond what you're thinking of. read more