I should first say that I don't return fish unless it's absolutely necessary (emergencies like if a tank cracks and I have no where to put them etc) and I personaly research every fish I plan to buy before stepping into the LFS.
However, I know a lot of (particularly beginners) fish-keepers expect the LFSs to offer sound advice - and so they should - but unfortunately it simply doesn't work that way. Or they might place their trust in friends or family who claim to know what they are talking about. No matter how it happens, these people think they don't need to do their own research (through no fault of their own mind you) and end up finding out they got the wrong fish too late. Oftentimes, getting a larger or seperate tank or making the tank suitable for these fish is too costly or whatever and their only option is to return fish. I don't like this, they should have done research beforehand, but it isn't realy their fault and hopefuly they'll learn from their mistake and next time will be different...
On the other hand, there are those who are irresponsible and knowing very well the possible consequences, walk into an LFS and buy whatever takes their fancy on impulse. I don't think it is fair to play with fish lives like that or risk them just because they know they can always return the fish or because there's a guarantee or for whatever other reason. Unfortunately, there isn't much anyone can do about these people besides telling them again and again and hoping that it will eventualy be absorbed and they'll learn and change.
Last there are people who have kept the same fish for a very long time and are getting bored and want something new in their tanks but cannot afford a new one. I understand this and I'll admit there have been times where I have even considered exchanging some of my fish for something else I realy realy want. I never have. I simply cannot take back a fish I have owned for several years, leaving it at an LFS with no control over who eventualy gets it, and replace it with another fish. In my opinion, if people want new fish, they should take the time to re-home their old ones with people they believe will take good care of them. We all know the sort of people who might end up buying fish from an LFS and I wouldn't want to take that risk with my fish' lives. Better yet, they should stick to their fish and let them live out their lives. Most people would never think of taking a dog to a shelter and then just replacing it with another and, IMO, people should think of fish in the same exact way. It may even teach people that maybe next time they should research their fish even more carefuly and make sure they want what they are getting and are prepaired for the full responsibility.
Sure, there are exceptions like in emergencies when your realy need to get rid of a fish or if you end up with an individual that is particularly aggressive or whatever and I udnerstand that people can change their minds and end up disliking something they already have and I accept all this and have no problem with it. Still, I think many people need to learn to value their fish more as living things and less as replaceable ornaments.
Anyway, that's just my opinion and everyone is entitled to their own so...