I didn't say they would go to jail. But people are going to find out who minted the bad coin(s). It's not going to be good for their business and likely will end their operation. It is also true that depending on where they live it could be illegal. For example selling such coins may constitute fraud in the U.S. Although it may be unrealistic to assume the police will help.
Personally I am against "colored" coins. However if we have a 51% attack I think we would have such a system within months. I assume nobody wants counterfeit bitcoins.