It starts and ends where there is a 'duty of care'.
The climbs on Rosea aren't specifically sanctioned by Parks, are not maintained by Parks, are not publicised by Parks, and it would be a nearly impossible task to actually go and remove all the loose blocks (well beyond what anyone would say could be reasonably expected of Parks) - therefore no liability.
The tracks around Rosea are sanctioned by Parks, they are maintained by Parks, they are publicised by Parks, it is clearly foreseeable that an accident could occur and the removal of obviously dangerous trees is a fairly simple task - therefore liability.
Tort law is often (ok - not always) more rational and justifiable than most people would give it credit for.
I must say I'm still not really convinced that falling trees warrant the closure - but having to remove fallen trees so that people don't go around and trample a whole bunch of the re-growth is a pretty good reason for keeping it closed.