You guy have to remember that they are a business and count on sales. If you saw a 20g tank with 1-5 fish, you would probably skip it thinking that there isn't much to look at and/or they have a poor selection. You might think that they are being responsible for not overstocking their tanks, but they are losing money if they do that. If you saw a 20g tank with 20-50 fish, you would be more interested in looking at it to see which is the best looker and are more likely to buy then just pass by. We are hobbyist and are expected to provide good care for a pet, you can't hold any stores to the same level of care as they are keeping animals alive for sale and not to keep for long term.
If it was me, I'd do the same. You have to maximize your space and predict sales. If I noticed that the 20g tank with a 100 Neons are selling faster than I can stock them, I would double the order and they would still go in the same 20g tank. I wouldn't split them in between anymore tanks as that would just limit anymore stock that I would be able to buy and sell.