Years ago, this wasn't the case, if you owned property in NH you could register your car/truck here and have permanent residence in another state. Then what happened was, it became popular for people trying to avoid high insurance rates in neighboring states. The result was that NH insurance rates skyrocketed as everyone living in NH was paying for out of state accidents and thefts of those who tried to cheat the system.
So, NH passed the 15 day rule to bring insurance rates back into line with what they should be, and the other states, who were losing registration revenue started looking for NH registered vehicles parked at homes of local residents.
If the law is tightening up again, I'll guess there is a resurgence of cheating the rules.
For example:
http://www.eagletribune.com/local/x1...e-plates/print