The problem is you have to accommodate for the lowest common denominator. With my new shiny EA Hat on, if the debate is banning offroading on byways, then you just need to see the mess that illegal offroading has done to the local byways. We have an stone bridge, ford and nice little byway round this way that has been trashed by 4x4s water splashing and hacking their own paths. How can you differentiate between someone who will tread lightly and someone who has no care for the environment? These activities cannot be easily policed, so it's easier to just ban everyone.
Now Urban 4x4s... Even with my EA hat on, I like my landy, and I want to keep it. I pay a premium for the fuel I use, and I don't see why they need to be shunned aside.
(BTW I didn't email this!)