There has been a lot of debate about the affect the aquarium hobby was having on the world's reefs. (sorry! here I go off-topic again...

) Actually, as you were told by the person at the store, much of the
live corals sold in the aquarium trade today are either captive bred here in the US (many by the hobbyists themselves...me included!

) or are "farm raised" in places like Fiji. It is very rare now for divers to actually remove live corals from the reefs since it is more difficult, more expensive, more risk of loss, and more illegal. Many countries ban collection in their territorial waters. Also the hobby itself has put a lot of effort into teaching native divers and collectors less destructive methods such as aquafarming and aquaculturing. Check out the
http://www.amdareef.com/ website for more info.
The greatest threat to the world's reefs are being caused by global pollution and its effects on the environment (ie. global warming). Also the increase of tourism, causing over development of coastal properties, thus increasing the sewage and toxic and chemical runoff from land. Port destinations are dredging the harbors making them deeper for the bigger cruise ships to enter their ports, destroying millions of acres of delicate reef in the process. Compared to all of that, the hobby trade's impact is almost insignificant.
The nice thing about the saltwater reef hobby is that it is so expensive and complicated. It's much harder to jump into the hobby spontaneously and most of the pet stores that are able to keep their corals alive will provide decent education to help people get started.