Sadly, people don't know that they are buying artificially colored fish. They go to the store, see cute little rainbow fish labeled things like "jelly bean" or "fruit basket" or "easter" etc. They aren't experienced fish people or don't know about the dyeing since the fish aren't even labeled as such, no warning, they're just labeled with cute, colorful names. So they buy these "cute" fish and the store is enticed to buy more since people are unwittingly buying them. I'm surprised to see that some people here on the forum don't know about the method of dyeing, but that goes to show that your average shopper won't have the slightest clue what they are buying.
I've seen tattooed cats before though... This stuff doesn't stop at fish.