I'm just guessing here, but I would imagine it is so they are not subject to being a "commercial vehicle" for inspection purposes. Some states (such as CA) you are subject to a search if you are a commercial vehicle.
I was hauling a boat for a buddy from CA, and because it's wasn't a commercial haul, I could breeze through the regular check point and didn't get pulled over for further inspection.
I think in some states, commercial vehicle inspection is mandatory.
But take this all as just an opinion and my own reasoning, just because I've asked myself the same question.