I think it's because local laws with respect to tinting are so varied here in the US. Some states don't restrict it, others do. Here in PA, the law is written such that you're allowed x% of tint on the front (door) windows, but it's TOTAL tint, including the tint of the glass itself. So anything you add beyond the standard glass tint technically tips it over the edge.
If there was a factory-applied tint that can't be removed, then there's a chance the car might actually be illegal in another/adjoining states. I'm pretty sure lawyers would get involved if it ever impeded the resale of the car.
In fact, along these lines, many manufacturers now build all of their US-market cars with emissions that are certified for the State of California for this very reason. It used to be an option for the local California markets. Now the rest of us are stuck with it too.