Okay....here's the deal with Jeeps vs everything else: Jeeps have the doors designed to come off by the user, thus they are allowed to be driven with them off. This is due in part to the lip in the door well, as well as other designs. If the doors are taken off, then adequate mirrors have to be left on the jeep still (usually, this is two total mirrors either combination with side or rear view). On some jeeps, the windshield is also designed to fold down....
All other vehicles are not designed this way. That's why it's against the law to drive other vehicles this way. I can't say what the legalities are to custom/tube doors, but I can't see there being a problem unless the police officer you get is either wanting to pick on you, or having a bad day.
Heck, I've thought about doing a set of custom cut doors someday...
