> The issue is that they, and other administrative organs, often change their interpretation of what the law "has always meant".

Yes, and if that conflicts with what the courts believe the law has always meant, those decisions won't survive contact with the legal system. An ex post facto law is a law creating (or enhancing) criminal penalties for acts that exist before the law is passed. Changing administration interpretations are like changing prosecutorial priorities (and the former comes with a lot more notice and specificity than the latter) -- they only have effect so long as they are within the bounds of what the courts will accept was covered by the law when it was passed.

The courts will accept a hell of a lot, due to Chevron deference. Statutes will use a term (eg, "readily convertible" or "replica"), and the administrative agency will decide this term means different things depending on the day of the week and which party is in power. All it has to be is "plausible", not consistent, and the courts will defer to their interpretation.

https://en.wikipedia.org/wiki/Chevron_U.S.A.,_Inc._v._Natura....

Just a note, there is a possibility that the person you replied to is comfortable with circular logic about why the behavior is not controversial under the supposition that "the law is the law." This may be a semantical discussion about why it is not "retroactive" in a legally damning sense, despite the similarities of the distinct ability to civilly and criminally sanction somebody for something they did in the past. Long before representatives of the government decided that person's prior actions would fall under their jurisdiction.