If the approach is, 'we would like to hire you. . .oh but by the way we're not going to pay you,' that's different from, 'we were wondering if you might like to enter into a mutually beneficial partnership with us'.
I suppose it's a question of the nature of 'pay' and whether it has to be direct or indirect. If professionals think they will earn more money long term from an engagment then it's not unreasonable for them to agree. But they would be well within their rights to say no, especially if the approach more resembled the first example above and not the second. Again, I don't think the dangerous precedent argument works even for professionals in this case due to the uniqueness of the event.