Yes
>Was the not joining an option for either of them?
Yes
> From what I know you cannot play in NBA nor you can play in Hollywood w/o being a union member.
That isn't true. It is much harder in Hollywood than in the NBA, but some people decide to not join or leave the union. Jon Voight is the most famous example that comes to mind. He continues to work despite quitting SAG-AFTRA. It is also more common for actors who live in cities where non-union productions are more common (outside of LA and NYC).
Can you expand on this, what is "harder" exactly? They don't enjoy the benefits of the union or they cannot get high paying roles? Also, are you implying Jon Voight is not a SAG member, if so, I'd like to see your sources because from what I can find he is a member while being in dispute with it (also shows how great the unions are).
And as far as NBA goes, not being a member is irrelevant as you are still paying dues and obey union regulations, fun fact I've found looking for this: Michael Jordan was not a member of the union and does seem to do better than LeBron James, e.g. there are those "Jordan" shoes which seem to do much stronger than "LeBrons".
It’s disappointing but not surprising
Some of these people truly, truly believe that they are so gifted and so talented that the work opportunities will simply never dry up.
He's definitely been underpaid in the past, but I'm not seeing it in his current contract.
By what logic?