That's a very good question and there is no definitive answer. My thought process is doing that can cause a lot of stress to the fish and to me it's not ethical. If you buy a fish that may thrive in your tank for a few years before needing a new home I can see the argument. In this case the fish in question will get huge and grows fast. It will "outgrow" a 36 gallon in a few months.....but actually it will not be stress free in a tank that size at any time.
As far as the LFS in question if they recommend that fish in this size tank...they've lost their A rep.

I don't know one that recommend it in a tank twice that size. LFS in general are a poor source of that kind of information in my experience unfortunately.
Wish you the best of luck and just want you to have the best chance possible of being successful. This can be a hard hobby with so much conflicting information. I can remember the confusion as a new reefer, despite my education in biology/zoology.