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