In Canada, it is not. If you don’t join the union, you may still benefit from the union’s bargaining power so you will pay some dues to the union. I would presume this varies per region.
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.