Because those non-union workers are enjoying the benefits union workers fought for.
When I was a college kid the steel mills around here were foing great guns. They offered college students summer jobs in the mills. The pay was fantastic, far more than a kid could earn at a Dairy Queen or a hardware store. That money helped out a lot. And I was grateful to kick in a portion of my pay to the USW so I could earn enough to pay for school.
I'm not sure if you can see this or not, but that argument is really weak.
Unions helped bring about a 40 hour work week decades ago. By your logic, everyone in America should pay the unions for that. The same for anything that helps other people.
We all should be paying dues to the phone switching company, the auto manufacturer; hell, the inventor of velcro or a thousand other things that help everyone.
Sorry, but it is un-American to require people to pay protection money. That's why it is against the law.