Is it mandatory for employees to join the union?
Unions are weak because the deck was stacked, the same way it is in every kleptocracy.
And it's not really "scummy" of the union that health insurance had weird loopholes for kids.
In many industries it's a requirement of the bargaining agreement. But many states have passed 'right to work' laws which state you can't be forced.
And still, I know of positions where there are 'union' and 'non-union' people basically doing the same job at companies. Non-union usually comes with a pay bump to offset not having the union benefits.
In a world where it isn't illegal to fire everyone who even thinks of joining the union, you might have some (ethically wrong) point. That's not in this world.