Around 1999, a cyclist in Green Lane Park here in SE PA decided to go riding on the ultra-techy orange trail without a helmet. He crashed, fractured his skull and then lay there and died because he was all alone. After that, bikes were banned from Green Lane for the next decade plus. I'm not sure why there was ultimately a change of heart, but the fact remains that bikes were effectively banned for all of the 00's (some folks still rode there from what I understand, but myself and any of my riding buddies avoided it until a few years ago when we found out about the re-opening.)
I'm not personally aware of others, but you don't need to actually have another case to see the logic in the argument - a land manager has to make choices regarding risk tolerances. Given the relatively small operating budgets of many parks and trail systems, it's a pretty easy decision for many of them to exclude bicycles from those systems simply to avoid the possibility of a crash resulting in litigation. If they can't enforce an ironclad waiver (or guarantee that every user will see and sign that waiver) then even in limited liability venues, the potential loss due to litigation could destroy their budgets. If some f*cking knucklehead wants to ride a technical trail without the sense to protect his head given the very real possibility of a crash, that knucklehead presents a level of risk that any land manager would consider intolerable. And since they can't selectively ban individuals unless they pay to have a sentry posted at every possible access point, it's much more cost effective to simply ban everyone.
The simple fact is, a rider without a helmet presents a higher perceived risk for a negative press event and/or litigation for a land manager than one with a helmet. (The argument that a rider with a helmet will have a false sense of invincibility and will take more chances is nonsense on its face - personal risk tolerance is a psychological component of an individual personality, it isn't created by specific opportunities. If you've always been cautious about your safety, for example, you don't change your personality because someone gave you a helmet.) And as long as that higher perceived risk exists, helmetless riders can only increase the likelihood of a park or trail system being closed to riders.