The unions have power in the industry and they work to maintain that power. That includes trying to establish rules making it difficult for union members to work non-union gigs and vice versa. This gives incentives for studios to make their productions union jobs which strengthens the union.

An actor who refuses to join the union will have a harder time getting work because people generally don't want to deal with these headaches. However, actors can certainly have careers without being in the union. This is easier in places like Chicago or Atlanta were there is still a decent amount movie/TV production, but a lower percentage of the work is union work.

Jon Voight is a "fees-paying non-member". He is not part of the union. He has to pay dues whenever he wants to work union jobs.[1] Same as your NBA example.

I think you are mistaken about Michael Jordan. He opted out of union's shared licensing agreement[2] which is just one of the many benefits of the union. Have you found something that says he completely left the union?

[1] - https://www.backstage.com/magazine/article/voight-gets-nom-i...

[2] - https://www.nytimes.com/1992/02/01/sports/sports-of-the-time...