It seems alot of the replys say something to the affect that as long as the hammock is hanging from trees within the campsite, then it is probably acceptable, but depends on local authority.
My question is what constitutes the boundary of a campsite? Any campsite Ive been to does not have boundary lines between it and the neighbors. The best trees to use in established campsites that I have found tend to be on the perimeter of the "open space" of the site.
I think boundary issues of useable trees is going to be a pretty big question if its determined that hammocks are allowed in most of these parks.