Then why can't laws target only the illegal riding instead of spilling over into the legal riding? Why is it that those who claim to target only illegal riding always advocate closure as a management tool? Closure only hurts the legal rider, not the illegal rider.
When trails are outlawed, law abiding riders avoid them (with the exception of the obviously ignorant). Tweekers, teenagers, and intentionally illegal riders will be the only ones using them. Legal riders will go elsewhere until there is nothing left.
The backlash against any laws or litigation that makes legal trails into illegal trails is due to the lengthy history of dubious closures. Some places now closed to OHV use needed to be closed to any and all human use. Some needed to be closed only to vehicles and mechanical travel. Some needed to be closed to horses as well as mechanized travel. But there are many areas now closed where vehicle use has been historic, well documented, and benign to the environment. Yet, closure was forced simply because some people with more money than common sense simply want all OHV use outlawed, and want us all in high rise buildings in downtown San Bernardino. OHV users are tired of large areas being closed to OHV use simply because someone has decided that the activity is one with which they don't agree. 35 years of abuse of OHV users has taken its toll, and we are also frustrated and unwilling to sit still for being used as the whipping boy.
If you can manage to pass legislation that deals with the illegal riding without infringing on the legal sort, you'll likely see no significant opposition.