fish shops sell fish which become far too large for the home aquarium for one simple reason ... ignorant people buy them and that translates to money. very few fish shops actually know anything about the fish they sell. and worse, they don't care. they're in business to sell fish and once those fish leave the fish shop, they couldn't care less if the fish lives or dies. in fact if they die, all the better since you'll be back to buy more.
other shops as the article suggests, are only too happy to sell you more fish if you don't manage to kill your initial purchase(s) first. they're also happy to sell you lots of medications for the various diseases your fish succumb to due to lack of knowledge on your part, and lots of crap you don't need such as pH adjusters and other bells and whistles. understand .. i'm not talking about YOU specifically but rather, the generic 'you' the hobbyist.
the only way to combat this phenomenon is for you to DO THE NECESSARY RESEARCH BEFORE YOU BUY THE FISH, RATHER THAN AFTERWARDS WHEN IT'S TOO LATE. if you don't buy that arowana or pacu for your 30 gallon tank, they'll stop selling them.