I guess this question is for Michael more than anyone else.
I came across an interesting summary of permit policies regarding photography in National, State and Local parks in the US. www.calphoto.com/policies.htm
The policies for commercial fine art shooting in national parks are liberal. But I was shocked to learn that to shoot in California State Parks one needed a permit and a certificte of liability insurance, even if your purpose was still photography for fine art. You need such a permit and insurance to shoot in San Francisco's parks, and parks in Los Angeles as well.
Since you sell your prints, do you carry liability insurance? Has this been an issue for you in your career? I understand how local or city parks would want to restrict tripods. But it seems outrageous that if my purpose was to sell a fine art print of a macro shot of a flower in a San Francisco park I would be violating the law if I did so without a permit and a 1 million dollar liability policy!