I'm not saying it doesn't happen, just that I haven't come across it at work.
When new to fishkeeping, it is always quite daunting and people want what's best for their fish. It's always best to start out with stocking your tank within the guideline so that you can get used to maintaining the tank with as few problems as possible. People have to get used to testing their water, feeding routines, tank cleaning, filter cleaning etc. This in itself is quite scary when you are new to it all. They also have to take into account the size of the filter and how much waste it can handle.
Once they are used to doing this, they will then learn to stock accordingly, depending on tank size, filter size, fish size etc.
It's always best to learn to walk before you run...