You seem to be a jury of one with that response, as everyone else has said they think the fine was unreasonable and probably unlawful. I got the story from a friend, the person concerned is her son, and from what I know of him none of the suggestions in your final sentence would have applied. He is in the military so familiar with adhering to instructions and using common sense. The reason the wife went with him was so she could have a quick chat with her mum from the driveway via a window while the mother stayed inside and the medication was left on her doorstep, as she hadn't been able to see her for several weeks. All sounds perfectly reasonable to me, but perhaps the police officer was not amenable to seeing the bigger picture that day.
Incidentally I tweeted the Home Secretary (@patel4witham) asking the question. Who would like to guess whether I had a reply?