Hi,
I used to work for Petsmart, and their official stance is that they don't condone the illegal harvesting of corals and live fish.
Truthfully, it's more along the lines of costs associated with maintaining them in their stores. Most, if not all Petsmarts, tank systems are all run off of one major filter unit. So we all used to think that it wouldn't have been cost effective for them to sell saltwater fish. That and the fact that they are harder to keep...
I used to hear that some stores down south, like in Texas maybe, would have little display type tanks of saltwater, but that's about it.