Is this really true?
I know our local Trail Riders Federation (polite, controlled motorcyclists who do a lot of work to maintain BOATS and assist at LD rides) and their members were very disappointed a couple of years back when the BHS declined to support them over some access matters, after they had always cooperated with BHS requests for evidence of use in right-of-way enquiries.
TBH I don't see the problem in APPROPRIATE use of byways by considerate motor vehicles. However it has to be taken on an individual basis for each byway, and there need to be much more stringent - and easily enforceable! - penalties for those who break the law or breach traffic regulation orders.
Up here, if it weren't for the use of byways and green lanes by the TRF, many of them would be totally impenetrable or even worse, lost for ever.