Try managing 50-100 tanks and serving at the same time...
No excuse - there should be enough staff to cope especailly with the welfare of the fish. The trouble is most fish shops just look at it as a business and dont really bother too much about the welfare of the fish - they are cheap enough from the wholesalers.
If shops did care they would give out more information on the care of fish and not just a quick sale!
One of the things I'd like to see is a simple fact sheet on each tank for every fish in stock. Max size, suggested aquarium size, aggression, temperature and things like that. It's dead easy to do and wouldn't cost much, seeing as you could even do it at home. A simple 'caring for' fact sheet too covering the basics would help thousands of people out, as a lot of people just decide they want fish, go buy a tank and sling some in...
Granted, some people genuinely don't care, but those that do would appreciate it and you just might bring someone else into the hobby, rather than them give up after a couple of months as their fish keep dying. I can't see any downsides to this and it's not like you're going to have to explain to customers all day, taking up your time for maintaining the store.