How would you feel if your governement makes a no mixer law and backdates it so that you have to explain your involvement as an advertisement service for them? and user of their service as you just admitted to.
If your government punishes you for something that wasn't illegal at the time, you have much bigger things to worry about. It would be a Human Rights violation:
Article 7 No punishment without law
1 No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence under national or international law at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the criminal offence was committed.
2 This Article shall not prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according to the general principles of law recognised by civilised nations.
This means that:
The right to no punishment without law is absolute. This means that it cannot be restricted in any way.
So to answer your question: if my government would do what you suggested, it would be time to bring out the pitchforks.