Dealing with the general public in this regard as a pet store employee.. I can say with all honestly that I always warn people about the sizing requirements and long term care as well as informing them what often happens with fish once they reach maximum size. Sometimes the customer will exclaim and go "not for me then!", but a whole lot of the time it doesn't make one bit of a difference and they want what they want. I wish I could always refuse a sale, but there are so many ways they can get around it anywhere from "Well, the smaller tank is what he'll be in until I get my 500gallon cycled.." to denial that it will actually get so large in the first place.
One such case of this was when a gentleman brought us a rubbermaid bin on wheels with his pacu informing us that we would indeed take back his monster fish because we should be ashamed for selling something that grew so large. The kicker was that I remembered him from several years prior because he was one of the first customers to ever yell in my face when I tried to merely warn against it in the first place. That has stuck with me and I really feel like you can only educate the masses so far. Yes, people impulse shop. Should we? No. In a perfect world we wouldn't give in to temptation. Unfortunately people always will.. and it's certainly not something you can blame on any one person. Smart consumerism as well as wise pet ownership is something to strive for.
I'm sure there are plenty of people in shops across the world who don't bother to warn people, however. But maybe, just maybe, they are tired of arguing. On a positive note, we no longer sell them, thank goodness!