I'm of two minds on this one:
1) Don't think the Mazda3 was tailgating. Tailgating (aka "riding someone's ass") is really tight and leaves no room for the driver in front to brake safely. It is a very aggressive following maneuver that is only done by idiot douchebags who don't think about the consequences of their driving, even if the person they are tailing is going too slow.
That said...
2) The Mazda3 was clearly playing with him and matching maneuvers with him, ie speeding up or slowing down in sync with the BMW. That can come off as very aggressive and if you're in the car in front you have no real way of telling if he is just playing around or if the driver behind you has negative intentions. What if it was a woman behind the wheel? Would you think some guy is following you? If you STOPPED and waved them by how do you know the guy isn't going to stop to and make things worse for you? For an example of how this feels from the driver’s perspective read this post from this very forum:
http://www.1addicts.com/forums/showp...5&postcount=67. Yeah the poster was probably being followed closer than the guy in this video but I think the idea is the same.
I think that as the audience we're aware of the Mazda driver's intentions (or at least how he can paint it since he's the owner of the video and is essentially writing history here) so it disarms us but as the Bimmer driver you wouldn't know and it would make me a little peeved too.
What I would do is actually slow down and use my BMW Assist/Bluetooth to call the cops and lead him right to them rather than stopping in the middle of the road and making things worse. But I see two bad drivers here, not one.