Large fish aren't the problem, neither are the LFS. The customer who buys a fish trusting the word of a minimum wage employee and not trying to find out about their fish ahead of time is the problem. The truth is too, in a 2'x2' tank, chances are your fish won't get bigger than a foot, be it stunting or premature death. What sizes should stores advertise? Record sizes? Typical captive sizes?
When customers want fish that are tank busters, I usually would try to tell them how big the fish get, and they would say "Not in my tank!" laugh and have me catch the fish. Or I would get the occasional "I know" or "That's why I want it!". Many times, customers come into my LFS and ask for pacus or ID sharks. We explain why we don't sell them, and then they ask "Well who does?" and just head over to our competitor.
If a customer wants a tank buster for their 55, not much is going to stop them.
Where I work, we always try to steer customers in the right direction regardless of money. We pass up many sales because customers couldn't find a RTC that stays small, or didn't know aquatic frogs need filters. However, more often than not, the customers just don't care and buy it anyways. It's great to see everybody blaming greed and LFS' desire for money when I bet none of them have worked at a LFS.