I think it's a situational thing. Most of the time, it's idiots who, as stated, have no actual plans to re-home them, and the fish ends up dying long before then, anyway.
I've attempted a watered down version of this in the past. I used to have a 30g tank with a yoyo loach pack, and a few times I brought home oddball clown loaches from Petland with the intention of growing them out and then selling/trading/giving them away. It wasnt the desire to flip them for a profit, just the desire to try to give that one individual fish a better home than it'd get otherwise, at Petland or in some idiot's undersized tank. However, I became that idiot when, both times, the clown loach randomly deteriorated and died, despite all the other fish in the tank being totally healthy. The clowns were accepted by and schooled with the yoyos, so it wasnt the fact that they were kept singularly... just bad luck on my part, I suppose.