When’s it OK to ask musicians to work for free?

For sure it’s not when the person asking has raised $1.2 million for her new album but doesn’t want to pay back-up musicians on the road. Fortunately for all concerned, she (very grudgingly) changed her mind after considerable public outcry.
Many AFM locals had a prohibition in their bylaws about members working for free, at least without the approval of the Local. The intent was both to prevent pressure from being put on musicians to work for free and to make sure that the principle that performing music should be paid work was upheld. And, when most performers were members of the AFM, that was a principle that could be enforced.
But fundamentally it’s a moral issue and not an economic one. My own view is that the only time it’s appropriate for someone to be asking for free services from musicians is when everyone involved is being treated equally; ie not being paid. That would include anyone working as staff or stagehands or anything else. I’ve asked colleagues to perform with me on a benefit for a non-profit; in that case, the only financial benefit was to the non-profit. But if one or more participants is/are seeing either a financial benefit or a career boost that other participants aren’t seeing, it seems unfair to me that the rest should be asked to subsidize that benefit by their free labor.
I’ve often suspected that the reason that the people who become conductors actually get to develop their skills at the beginning is because they’re ones with the chutzpah to ask their friends to become a practice tool for no pay. That would explain a lot about some conductors I’ve known, including why some of them aren’t very good. Conductors are a self-selected population, and that selection process requires some character traits that have no connection at all with talent or musicianship. I wonder just how much conducting talent lurks unrecognized in our field because those in who the talent resides aren’t pushy enough to get experience.