What an aquarium or zoo can keep or display is not equivalent to what a private person can buy to own. Those are separate things. You might as well ask if it's illegal to keep a California condor then how does the LA Zoo have them.
Haven't been this year but the Steinhart Aquarium doesn't have blacktip reef sharks, they are also cheap in comparison to other fish the academy owns, they once had a group of three peppermint angels which are valued at around 30,000 dollars each or about 15 reef sharks.
More and more aquariums are making efforts to captive-breed blacktip reefs in house, and a few have succeeded, but the vast majority of new additions are going to be wild caught. And yes, as everyone here has demonstrated, most large public aquaria have more than enough budget to manage purchasing them, though they also typically do so from reputable sources that service the entire industry. Certain collection companies are shark specialists, there's one I know of in Australia that supplies sharks all over the world.
It's really odd that they're gone now, they were a pretty iconic species in the remodeled Philippine Reef Lagoon for a while. Steinhart now has only two shark species, and that tank feels quite empty. I feel as though they need to either reconsider blacktip reefs or get something else like a bonnethead for that section.
