I think the issue of legality stems from having the appropriate CITES and permits to collect and import into the market where you live. Also, as certain locals are becoming less open to having collectors come in and take fish from their waters to be exported for the marine trade, I don't think it's unreasonable to charge a good deal of money for these permits. It's simple supply and demand, and the more they charge, the less fish will be taken.
Additionally, many of these rare fish (not so sure if Black or Gem Tangs are included in this category) are found in areas that are difficult and/or expensive to access and sometimes at depths that are very dangerous for the diver.
Personally, I think it's perfectly acceptable for these "rare" fish to cost upwards of $500-1K. When you think about it, someone is diving to great depths, at their own risk, to collect a fish somewhere in the middle of the ocean and then is being shipped and imported into the US (or other country) generally via overnight shipping. I think that's pretty incredible.....
Lastly, no one said you had to have a Black Tang, or a Gem, or that deepwater Butterfly. If you're put off by the price, don't buy it, someone else will.