I have some of the very best fish stores where I live--and some I won't set foot in. But only considering the good stores, there are those who have some decor in the tanks, and those that have none. Both are acceptable, but both can be abused too.
Catching fish in a tank with any decor can be very difficult; not only does this waste time, but chasing fish around is more and more stressful on the fish (as well as the employee
) and may cause the fish damage in several ways. So the bare tank avoids this, and catching fish is quick and usually easy, as they can readily be cornered with two nets or a net and a hand.
When one considers what most fish have gone through from capture in the wild to transport and finally the store, it is miraculous that any even survive. A bare tank that is well maintained--water parameters, quality, low light--is probably less stressful than one might imagine. Light is extremely important here; my local professional importer and breeder maintains bare tanks but they have no lights; the ambient room light is it, and this makes a big difference to the fish.
There are some organizations attempting to introduce legislation in some countries to monitor fish similar to what exists (in some places) with mammal pets. Legislators often go overboard unfortunately, so this may end up being worse rather than beneficial.