Transports and passengers firing

I've always thought my raiders/venoms could move 12" and the passengers still fire at full BS, apparently this is wrong.. I've been told they can only move 6" and fire like normal tanks etc. I thought the whole point of it being a fast, open topped skimmer was that it could move around and fire. Can anyone clarify this?

http://www.thedarkcity.net/t4640-moving-and-shooting

^This thread has pretty much all the answers to this question.