You're totally missing the point. I have seen white mice populations, fyi. They're not like cats and dogs that you just see walking down the street.

People dump small animals all the time, trust me. They sell ALL their small animals this way (single sex). They only sell them as pets, no feeders, which is why they don't want people breeding them. I grew up having mice, as well as rats, as pets. If you take care of them, and clean them properly, there is no smell at all. Great child pets, too. As far as the tank, you can only stock SO MANY things that customers want. Eventually, you run out of square footage, and have to cut back on slow sellers. 30's are not a popular size. You can get any size larger and smaller than that there, you just picked the one size they don't have. Try to view it from a retail standpoint. You can't please everyone.
Not to mention, what does any of your problems with them have to do with trusting them for fish????