I would presume the long arm of the law?Curious: who's enforcing this in your area? In much of the west, it's either a licensed motor vehicle, or it's not. It's pretty much binary.
Some municipalities have parking restrictions for, say, CCLBs that might protrude into the lane when parked, but that's it.
Link.
However I cannot find the RSA that regulates it, and I've also see a 35mph limit instead, in some links. I thought I had seen the RSA in the past... can't find it now.New Hampshire — There is no restriction on how far you may drive the mini-truck from your home or company, as long as you keep it under 25 miles per hour.