I only know about the FS. The main beef is w/in the 4x4 community around here...have skills and tools but cannot use them w/o a certificate. This applies to pre-opening of trails or 'official' work days where the FS has something to do with putting it together. Some clubs that sponsor trails are stuck with the same rules as well. If I'm on an open trail and a tree is down...it goes away If I lived anywhere near where they do the certifications I'd probably go but its a 2hr drive for me to any of them. I'm assuming it has something to do with liability.
You are not officially volunteering when you cut out the occasional tree. Quite a bit of the motorized trails around here are opened up by the first rider in the spring. The minute you take part in an organized, sanctioned work day, the certification requirement kicks in. I've driven 3 hours one way to go to volunteer training and pulled a trailer to stay in. Most everywhere is at least 2 hours from here.