I don't.
Retroactive enforcement is very unethical.
Take driving for instance. When you speed, you know you might get a ticket, that is fine, you are apparently willing to take that risk and if you get pulled over and ticketed you will probably end up paying the ticket. Around here, people are ALWAYS speeding on the interstates. 55mph, most drivers drive like 60 is the legal minimum and it's not that rare to see people speeding past you even if you are going 70.
Just imagine what would occur if the police decided to start monitoring people's speeds, recording the speeders for a year, and then retroactively ticket them all for every day the were speeding. Average ticket of $60, 400 per year, $24,000 in tickets for each and every commuter who drives on i495 twice a day. Do you think that would go over well? Do you think that is ethical?
Laws are not well known or understood by most people. Enforced laws are remembered, but an unenforced law might as well not exist.
Intentionally leaving a law unenforced for years and then going back and retroactively penalizing everyone who broke the law in the past is incredibly unethical.