Where I fail in understanding in how your lfs did quarantined the fish prior to you having it is. The majority of the lfs aquariums are that any number of tanks be on one system. For the most I believe they did was too housed it alone in its own tank. The other be is that you still would had put this fish in a stress situation by bagging it, in order to take home. So this method I would have to say would mean that I still would housed this fish in my QT, before ever placing into the main tank.
Perhaps this idea is of my own personal view in the matter, I do know just the same that your lfs still not doing anything that be far greater then ourselves in housing these fish in a QT as your first step. Another thing in this is, when your ready to take that fish home, it would gone through a great deal of stress all over again. The stress in this in your home QT is far less, for you would not be bagging the fish for transporting it.
Buddy