I’d add that from the park service’s perspective this is pretty simple. They don’t want to have people getting stuck in the backcountry or damaging trails, and as an agency with limited staffing, millions of visitors, and huge territory to protect it’s understandable that they do not want some complicated argument with every wannabe rules lawyer who thinks they can drive like they saw in the commercial. “Only 4WD” is a policy that doesn’t require training thousands of rangers around the country about which models are considered acceptable.