What they did or didn't do 6 years ago is irrelevant. And that is an ideal that is built into our justice system. If you get convicted for something that you did 6 years ago, you will get the penalty that was relevant at the time you committed the crime, not what it happens to be at the time of sentencing.
6 years ago the penalty for not registering a dog was a fine. Now it means the death of your dog if it happens to look like a pitty. The only thing that matters here is whether their dog was registered when these laws came into effect. And they tried to do register the dog before that date.
By all means, charge them the fines for those 6 years that their dog was not registered. But what happened in those years before the new laws took effect does not justify the harsh punishment and from a legal (and moral) perspective has absolutely nothing to do with it.