you're entering a whole new category of laws.
short version. (not 100% accurate, but enough to give you an idea).
if the vehicle came from the factory with a removable top, then you can remove the doors. if i remember correctly (researched this years ago when i was in a jeep off-roading club) you "technically" can only have the doors off if the top is off. but i've never seen or heard anybody being harrassed about that as jeeps "usually" the only vehicles driving around with their doors off.
probably also has something to do with the fact that (as far as i know), only jeep wranglers came from the factory with quick removable doors, where broncos and other removable top vehicles didn't. otherwise they probably would do. it just sort of became a "tradition" that is associated with jeep wranglers.
PS:......you could also in theory take a non-removable top vehicle (think jeep cherokee, or pickup truck) and replace the doors with tube doors (just as it sounds. only a tube going from hinge to latch, usually only seen on jeeps, but easy to build for any application) and still be legal.