1. fish turn to the sand for a food source and some for protection but not all fish "need" sand, however it will ofcourse add to a more natural, secure feeling enviroment. infact i know a few who keep tanks with rock sitting on glass bottoms. they do this so they can increase their water flow without creating a sandstorm which keeps un-eaten food and poop in the water which can be easier skimmed out which creates an ideal coral setting ( high flow rates with cleaner waters ) but no substrate allows for less stable parameters. back on track, gravel IMO should be left out of saltwater tanks and even crushed coral. the larger grain size allows for more food and waste to fall in, and get trapped around all of the rocks on the bottom. this can be cleaned to a certain extent by using a gravel vac, but can be completely avoided by using a sandbed, pref. a deep sand bed.
2. they prob. have a sump with bio balls, the exact setup you dont want to create. food and debris gets trapped in the slow flowing sump with bio balls, builds up, breaks down and leaches excess nutrients back into the water, usually causing bad algae outbreaks. IMO/IME the most effective method of a saltwater tank is with a deep sand bed and live rock and over powered skimmer. the live rock doesnt just act as a place for beneficial bacteria to thrive, but also creates "caves and tunnels" for fish to seek shelter and claim territories.
i could be completely wrong but since this is in a store these tanks are most likely because theyre "pretty" or because they attract customers which would be a shame, because that would prob. also mean there is no interest in the care of the actual livestock, just the viewing pleasure it brings. infact i wouldnt be shocked if a company comes in to "clean" this tank of a regular basis and i would be even less shocked if they were just replacing dead fish with new ones as theyre spotted. which is a complete shame but truth with how some companies and coorporations use their tanks. again i could be completely wrong, but had to air it out there.
im hoping this isnt the case and they arnt allowing you to take your pictures because they actually do care about the livestock ( even though how they have it setup doesnt reflect this ) . . .
next time you go in there and they still wont allow pictures, see if theyll allow a water sample