It's possible many clerks don't know better. But purposeful rearrangement of the items in the display cases happens all the time---and wares of different stones are often put together. It's much easier to put the goblets in each of the cases then assign someone to keep them topped up, rather than have to group the humidity-sensitive stuff in one place. Goblets and water are cheap and the water doesn't hurt anything else, so why not just put it on autopilot and make a single rule "water glass in every case, and it's Zhang's job to check and fill every morning before opening time..." Foreigners always seem to want to complicate things too much.