I found something thay says when they started the rental registration program.
http://www.coventryglenrealty.net/public/CSRentalPropertyRegistrationNotice020809.pdfAccording to that, they started enforcing it in March 2009. They were supposed to have notified all the property owners back then. If they had his address and didn't bother to notify him any time until now, it'd seem like it's their fault, but I don't know if a judge would agree.
I found an interview with one of the city councilmembers claiming it's mostly to help them find out if there are more than 4 unrelated people living in a rental and to have a local contact for code enforcement issues, but as far as I can tell, it seems to mostly be a scam to try to force out-of-town owners to hire local property management companies. I'd be highly surprised if no one's making money for themselves or a friend off that deal. What I am referring to is their rule that there has to be a local point of contact, and it can't be one of the renters. If you own a rental house and don't live here, that would seem to imply you have to hire one of these companies.
I remember hearing a homeowner complaining to the city at a neighborhood meeting about being asked to fill out a rental registration form for a guesthouse on her property. I can't remember exactly what they told jer though. I think that if she's renting it out, she had to register, but if she was just using it for relatives who came to visit but didn't charge them rent, there was maybe some other thing to fill out so that property would be exempt.