With lots of money in their pockets. Unfortunately, just like died fish, there are people that will buy them and as long as there are they will continue to sell them. As Tokis mentioned, it most likely isn't illegal. Fish just aren't thought of as animals like dogs, cats, etc. Most people couldn't care less about fish and to the best of my knowledge, there is still no scientific proof that fish can feel pain as we do. Is it cruel? Absolutely. But there probably isn't anything that can be done about it other than complain to the manager or owner. The authorities most likely aren't going to get involved.
it has been proven that fish dont feel pain, they feel pressure changes. their nerve endings do not conduct pain, they conduct pressure.