I am jumping in a little late here, but I have been thinking about this conversation a bit. Please note that I am speaking here as an audience member, poet, and general organizer, not in any kind of official NPS capacity.
Anyway: I think it's fairly clear that the "cannot" you are talking about here was being used to impart a certain level of social responsibility. I am quite certain that anyone who told your team that they "cannot" say some things was not trying to say that anyone is legally bound or otherwise restricted by PSI rules to censor their language, and I agree that it would be inappropriate for anyone to imply that was so. However, I think it's appropriate to tell someone they "cannot" say certain things to indicate what is and is not appropriate behavior. For instance, to draw a very loose parallel for illustrative purposes, I understand perfectly well what people mean when I am told that, as a white person, I "cannot" use the n-word because it is socially and historically offensive, inappropriate, and cruel. There is certainly no law, rule, or physical impediment that prevents me from saying this word. But I can't say it just the same.
I don't think that we have ever met in person and your posts here have always been respectful and polite here, so I hope that you will take this next paragraph as a suggestion to communicate and question, and not as an attack on you or your scene. It concerns me that your approach to your artists' poem and the subsequent backlash seems not to be to investigate the language and content that so universally distressed the NPS community and slam family, but instead to investigate the legality of the poem, and indeed to clarify that your poets did nothing wrong.
To me, your questions come across as a defense of the poem, which I think undermines the difficult work some folks did by approaching your poets at NPS, as well as the even harder work your poets did by opening up to listen. I would encourage you to keep an open mind about the criticism that was offered. I think it's pretty clear that no one is being censored at NPS.