Here's what orientation means when talking about multi-base characters.

Rule Book page 15:

A multi-base character that occupies more than two squares is a larger base character. A larger base character is only allowed to be oriented horizontally or vertically; it cannot be oriented diagonally.

Well that makes more sense.

You still can't move back and forth though.

Yes, I can. I'm doing so right now, but then I'm not currently trying to perform a ram maneuver with a little plastic vehicle, so it's no big deal.

The reason vehicles can't move like me is because it has been established that movement along a straight path in HeroClix doesn't allow a 180 degree change of the movement direction.

Maybe they should amend the text to include the word "continues"?

"Give this character a move action; it may only move along a continues straight horizontal or vertical path and it can’t change its orientation."

I think that one word leaves for a lot less interpretation as to whether or not you can back it up. Orientation to me just means the way it's facing but in Heroclix that doesn't currently matter since I can chose any point of the base from which to make the attack or move (it's funny to think of the Invisible Jet firing rockets from its engine).

"Move in a straight line" is pretty clear to me.

I'd like to see a cops reaction when he pulls over a rules lawyer suspected of DUI and asks him to walk in a straight line and the guy moves back and forth in one direction.

Except in this case Move In A Straight line doesn't mind if you're technically driving your Batmobile backwards.

Edit - Your cop assumption is why good lawyers get guilty people off because there's some vagueness in the law/rule and why you need a decade of schooling to become a lawyer in the first place. A single coma or word can legally be ruled against you.