What always confused me about that case was why the IL law was struck down about curved mudflaps. Why not strike down the AR law requiring straight ones?
Suppose that 25 states required curved mudflaps and the other 25 required straight ones. These laws are burdensome to interstate commerce. Which states have to change their laws?
But to respond to the OP, I tend to believe that a particularly restrictive state law with regard to fixed equipment on a vehicle could be considered burdensome to interstate commerce. Wasn’t there a case where one state (Iowa comes to mind) prohibited trucks over a particular weight and companies had to take a long route around the state? The Court struck that law down. My google-fu fails me.
It does seem excessive that a state can effectively prohibit a vehicle from its roads. If I am a Maryland resident and desire to have MD-legal tinting on my vehicle, I have to now buy a second vehicle if I ever want to travel in VA? What is the rational basis for these vehicle tint laws anyways? It cannot be safety because it is legal to drive with heavy tint wrapped around your eyes (sunglasses).
I am all in favor of states rights and whatnot, but something like a vehicle driven temporarily in a foreign state comes close to an almost textbook regulation of interstate commerce. Could VA pass a law saying a car travelling on its roads must have five tires touching the ground at all times as a valid safety regulation?
What about Morgan v. Com. of Va.? US Supreme Court Opinions and Cases | FindLaw
I realize it was a segregation case, but it was 1946 when segregation was permitted. If it is too burdensome to require people to change seats when entering a state, why is it not more burdensome to make car owners change fixed features on their cars?
When I lived in Virginia as a lad I was taught that there are no worse drivers than those from Maryland. Maybe it was a warning ticket.
Driving at night. You can take off the sunglasses.
If you’d lived in Maryland you’d have the opposite opinion. Fact is, there are fuckwits behind the wheel everywhere.
That said, had the OP just stayed on his side of the Potomac like I do, he wouldn’t have this problem. 
At least where I live, it is the police groups that actively lobby for the tint restrictions (and the individual officers enforce them zealously). Their position is that officers are in danger if they approach a vehicle and cannot see inside clearly.