A lot of the time its the instructions in these tanks that are to blame - people blindly follow instructions as they have faith that the info provided to them is correct.
For example, I bought two small tanks from a Walmart style Australian store cos they were cheap setups - I wanted them for bettas, one is just shy of 5 gallons and one is a 1.5 gallon hex. They are the same brand, and each were promoted as being 'ideal' for goldfish. - The picture on the box for the 1.5g hex shows SIX fancy goldfish, the 5 gallon picture showed about 10. Each came equipped with a pamphlet titled 'The Beginner's Guide to Goldfish' which I was so disgusted with I think I tore it up and threw it out. It said something on the lines of you could keep up to 10 goldfish in the tank, and instructions to clean it out ABOUT ONCE A MONTH. Needless to say that its disgusting that a petcare brand would provide this information. I should mention too that this brand had a range of "tanks" (if you could call them that) promoted as being for bettas too - they were about the size of a mug. A divided mug. Yes, the sell point was - this tank is divided, so you can keep TWO bettas happily! Ugh.
Now I keep one betta each in these tanks, and no other inhabitants, in fact, I tend to think the hex is still on the small side - I swap these fish over at water changes so they both get turns in the bigger tank. But to think of all the people out there who would put this amount of fish in a tank 'because thats what the instructions say I can do..."