I haven't noticed that many others in this thread reporting similar incidents. I've been slapped on the arse while riding 3 times, I reported each one and the police responded to all of them. The best one was a kid up the road got a section 59 (Google it) when his mates slapped me (and the driver was obviously too close), he had his car impounded and crushed several months when he misbehaved again or so my contacts tell me.
But yes, walk down the High Street today and slap just one arse, and I can guarantee you that the Police would treat it even more seriously, do it from a car and the Police will treat it as a motoring related crime for some reason. They are simply not capable, or equipped to isolate the offence when a vehicle is involved. It's the same reason why it's one of the most effective murder weapons (albeit random murder targets, unless you know where you victim is going to be walking or cycling on any given day). Go at your victim with a hammer, knife or gun, then it's life inside probably. Do it in a car, even if you're completely at fault you can expect a fine and points and if really unlucky, a short stretch inside.
Funny world, not that I'm laughing.