The usual reason I hear is that it is their job to protect the natural resources.
Tree Climbing threatens those natural resources according to the rangers. The interpretation of what harms a tree/natural resources, is left almost entirely up to the ranger on site to determine.
Did you know that in a national forest, it is a misdemeanor to stray off the marked path?
So access to any national forest land is limited to developed paths created by the forest 'service'. No marked official path, no access.
Much of our forest land is closed off to the public. The forest 'service', opens or closes our forest land to public access at will.
The reasons given are: Budget constraints, inability to meet ADA (americans with disabilities act) requirements, protecting endangered species reproduction, areas under 'study', areas 'too dangerous' for the general public, etc etc.
While many of the reasons make sense to me, my personal experience with rangers has shown me that overall the relationship between the rangers and the public is one of 'us and them'. Of all the rangers I have come into contact with, the attitude is that the general public is a trespasser. A trespasser who, simply by being there, must be monitored and watched.
Hard not to see their point as so much of the general public truly do not have the first clue about conservation protection or even care about it. Sure, when asked most people will claim they care, but ask them to pack out every bit of trash they pack in, and they have problems with doing that (for example).
But, nevertheless, having a park closed because there is no money to maintain a porta-potty, or the parking lot cannot be patrolled regularly by the police, is a hard one for me to accept.
People who DO practice conservation and have a respect and understanding of what it means to protect our natural resources get lumped together with other members of the public who DO NOT practice these principals.
I cannot blame the ranger, after all it is too hard to be able to tell one member of the public from another just from looking at them.
Take me, for example. Here I am, going to climb a tree in the park. I KNOW I will take every care to not harm the tree(s) I am going to climb. However on tree climbs in the past, I have been with climbers who are great climbers, but nevertheless scamper up the tree and break off all kinds of branches on their way up. Or smoke a cig and leave the butt on the ground.
Even me, who supposedly understands how to be careful, will snap off a tender shoot in the tree, or trample some fern on the ground.
So it is a tough situation for all concerned.
My solution is to avoid any contact with the rangers. Get in and get out 'low slow and under the radar'. Small groups of people, minimal impact.
Gerry Baranek taught me that. It took me awhile to truly understand what he was talking about, but it is truly best to climb these trees with only very small groups of like minded people.