I don't know what the reason behind the official policy is, but if I were an admin and this were my site, here's how I'd view it, the implicit condonation of - and illegality of - street racing nonwithstanding.
1. You allow street racing vids.
2. Someone gets into an accident while filming such a video under the pretense of some sort of oneupsmanship.
3. It's found that bimmerpost had anything to do with the reason that the person was making the video. E.g. user1 wants to beat user2's time set in videox, and post proof of it on bimmerpost.
Then you're opening up a legal can of worms you don't want to. Seriously, a lawyer could have a field day with that I'm sure, although I'm not a lawyer.