I've often wondered if LFS' are at least partially motivated to not give the whole truth when it comes to fish that they sell. It's the whole good money after bad deal. If you go in and want to buy that bala shark and a 10G tank, but you were told, "well, over the life of this fish, you're going to continually need to buy bigger and bigger tanks, and you're going to need to buy 3 or 4 more of them to keep it healthy, plus you'll spend hundreds on meds, food, filtration stuff, etc - all told, you're looking at a few thousand dollar investment", many people would walk away and not get into the hobby, IMO.
But, if you sell them without giving the whole truth, they'll have to keep coming back. Bigger tanks, bigger filters, more fish [after all, if the fish dies, you've spent all this money, might as well buy another one], meds, etc, etc, etc. Some of them will die and the owners will be turned off, but there are some serious people who will become addicted, like me.