It's not just USA. We have similar issues in Australia too. Many "enterprise
agreements" (as we call them) specifically prevent volunteers doing work that is part
of the duties of a paid staff member.
And of course any venue can impose rules on you as part of the contract of using their
premises, whether there is any law or any union agreement underpinning it. They may just
do it to protect themselves in relation to workplace safety. A volunteer may not know they
have to cover a cable snaking across the floor with gaffer tape (for example). They may
bring their own electrical cable without a recent "test tag" on it. Etc.
Kerry