As mentioned the state has the final say so, not the FMCSR.
In Michigan, I have yet to get a straight answer on the question form my different sources. Some say it is not a problem while others point to the existing laws, the full tinting of windows is supposed to be obtained by a medical waiver. It does not matter what the vehicle is, commercial or private.
This, by the way has caused a lot of problems with local LEOS because of falsified documents or false diagnostics of medical conditions by the urban crowd in order to get the tinting legally done.