I seem to be seeing this more frequently. It often appears on vehicles used to transport trailers used for motorsports.
I find it a bit puzzling. Are owners of such vehicles actually faced with people walking up to them in gas stations & rest areas, saying things like “I’ve got 10 tons of cement bags lying in the grass over here - what would you charge me to transport it to Seattle?” Or might it be a sort of affectation, along the lines of “Of course my vehicle is among those that are so desirable that hordes of people will want their services. We owners of such rigs naturally need a way to discourage this, lest we find ourselves spending all day frustrating hopes.”
Just WAG, but maybe a tax/registration/licensing thing- because the vehicles are similar in size and specifications to buses/OTR haulers, which are usually used to carry passengers/freight.
I’ve seen this on wreckers that are owned by a car repair business, used solely for their customers, and on wreckers owned by “recovery services” aka repo guys. I think it has something to do with taxing or licensing, just like Stan Doubt says, or maybe the level or type of liability insurance?
If I’m reading the code right, California considers any vehicle over 7,000 pounds to be a commercial vehicle for hire unless it’s carrying the sign; those operating such vehicles are subject to laws regarding financial responsibility (insurance and/or bond), safety (work hours, etc.), and drug testing. Other states can’t be entirely different.