Technically speaking, the requirements are specifically aimed at the number and location of rear view mirrors on the vehicle.
Indirectly, almost all vehicle codes specify the contingency for safety equipment (unspecified as to which equipment this pertains to). Any instance of removing safety equipment from a vehicle is deemed unlawful. However, as it doesn't specify doors in the safety equipment, it can go both ways. If you get a stickler for a cop, you might get a ticket. When it goes to court, it depends on the judge....but they'll usually stand behind the cops decision.
On a good note, there are several write-ups on making your doors removable. If you get a ticket, it should only be a fix it ticket, you can slap them back on, get the ticket signed, put them back in that giant pile of crap that some call a garage.
