ADV pass needed - assuming INSIDE the fence. But I am probably wrong. I find it ridiculous that I could take a vehicle on the dirt there, but if I park and walk - or park and ride, I need to pay. Ditto on everything LB wrote. Forgive me, I'm mostly retired and currently bored.
Even though history says they ticket there and have signed the area, my question at court would be "if the rangers wanted to secure the trailhead, where would they do that?" Being it's at the fence in the pic below, that would be where a reasonable human would thing "all things trailhead" apply. Without "between here and here" marked or "pay area" painted on the roadway adjacent to the cul-de-sac inside the fence, it's not clear.
The signs are there to inform people if they park within the fenced in area, they need an ADV pass. That's my (i'mnotalawyer) take on it.
Easements, etc. Your conclusion may be valid, dunno. There may be agreements in place but that'd take some time to research. SDPD had an agreement with the Navy for policing their housing areas - not just within city limits, but on base to a certain extent. I drove on Miramar past the guard shack to do a SWAT mission once. A weird feeling going on a military base for that.
But, their map shows the forest going to the freeway. This isn't the designated Bear Valley Trailhead from my reading, it's CNF, unless an ADV pass is needed to park along Hwy 80 - which I doubt.
The Bear Valley Trailhead map shows the road outside the fence line included.
Were these signs still there? No other indicators of where that applies?