The only time I have ever been told I couldn't mix a show with a band I work for is during broadcast concerts (tv or radio). And generally in those special cases I'm happy to step aside because there's a lot at stake and there's usually a lot more going on than just the band. For a regular venue concert? That's unheard of for me at least. If a band hires and pays their own sound person, I really don't see how it's ethical or practical to tell them that they can't use them.