Retroactive laws are already illegal in the United States, at least in the case of criminal law. This prohibition was put in place by the Constitution, which prohibits any ex post facto laws. These are laws that punish a person for something that was not illegal when the person did it.
In general, civil laws may be made retroactive. This practice should, in my opinion, be banned. It does not seem to be in any way fair to tell a person or a company that some action that they took in the past is now illegal or that they will have to pay damages for something they did which was not illegal.
Ex post facto laws are clearly unfair and should not be allowed in any situation.