Tact occurs in the absence of maturity, so immaturity is certainly adjacent to your superior term.

And it’s not so much about the person breaking out a soapbox, but more along the lines of breaking out a soapbox in a venue that is neither theirs nor public, especially when they are a contracted rep of that venue. That person was there on contract to entertain the patrons according to a clearly-defined program. That alone means they don’t air anything beyond what is defined in that program. It’s exceptionally rude to the patrons, who came there to see the program, not the person (specifically) or their protest.

Things get blurrier when the artist is the focus of the program, like when a band is playing, and they - as a group - pull out the soapbox. There I see a point in their favour, as patrons came to see them, specifically and exclusively, perform.

Even supporting artists have more of a leg up than this guy, because people will pay money to see a specific band even if it’s only an opener.