And sentinel values aren't just a problem of the system, but there would also be a human element. Even if you had a "no plate" option some would likely continue to write noplate. The current system might even support null values for all we know.
I just figured this is how people end up with license plates on their wall from their first vehicle etc.
However this is another aspect of my point - there is no clear uniformity on the relationship and handling of vehicle registration, plates, ticketing and enforcement. When things are poorly defined, you get inconsistent implementation and Bad Things(tm) happen as a result.
Considering just the wasted postage alone, thousands of dollars of public funds were wasted sending out obviously stupid and invalid tickets - totally avoidable if someone had stopped and thought about what they were doing.
Enforcement systems like this are always in danger of running away as a result and some schmuck being bankrupted or jailed (if states allow that) despite being able to demonstrate conclusively that they have done nothing wrong and are not responsible for any of the tickets. Public bodies are not good at admitting they are operating a fundamentally flawed system and it is always easier to ignore the small numbers of victims of their failure and hope no-one else notices. It does not take a lot of research to find examples of flagrant miscarriages of justice in these sorts of situations.
I always assumed other people ended up with a plate on the wall in the same way I did; by removing them from the car after your X drove it into a cliff-face and wrote the vehicle off.