In your analogy, the activity was illegal until the law was changed. If a patent is declared invalid, then it shouldn't have been valid when it was used in pros

In your analogy, the activity was illegal until the law was changed. If a patent is declared invalid, then it shouldn't have been valid when it was used in prosecution, so it's logical for the defendant to ask for some kind of redress. Whether the law actually works like that, I don't know.