I think a lot of it has to do with inexperiance in the world of fish keeping and a true lack of knowledge of the species of fish they buy. That, and I think we all know about the whole "Mines bigger than yours" thing!!!
I know that in Jan of 2002, I was just getting back into the hobby, and I wanted an Oscar damn it, didnt matter, I wanted one! Well by gosh I ended up getting 2 for my 20 gallon tank. They were about 3in when I bought them. By the time I got married in April, they were fighting a little bit, and by May, we had to get rid of one. Dont flame me for this, I know better now, But alas, he still is living in only a 45 gal tank. As soon as we get a house, or bigger apt, we are getting a bigger tank for the guy.
And as much as I love him to death, if I had it to do over agian, I would not have gotten him. He is just too big for the limited space I have right now. And if I could find a wonderful home for him, I would let him go. Just for the benifet of the oscar.
Most people buy these large fish out of ignorance/and or stupidity. I agree with you catfishmike, people who want to buy large fish, preditory fish and others of the like should have to sign a waiver stating that they understand the fish they are buying.
Hey, I really think that people should have to have a licenes to keep fish. And on the licsenes it states how big the tanks you have are, and then based on that you have a certian number of speices that the LFS can sell you. Also that would keep many new aquarist out there from spending butt-loads of money of fish just to have them all die because they thought that a sponge filter would be more than enough for the new 55gal tank they just bought.
Hehe, they should have virtual fish tanks, where you can select the fish and equimpent you keep.
EDIT: Also on the point of people dumping there fish into the local ecosystem, when a fish dies, or is goten rid of, keepers should be required to fill out movement forms, stating when and where a fish was sold, traded, or disposed of. And when people didnt comply with this, then the tree police would fine them money for potentaly releasing a non-native species into the local habitat. That would get some of the more careless keepers attention pretty darn quick and make them think twice about doing it.