I'm in a union here in Germany. Why am I supposed to pay up for the folks in the US who still act against their interests and refuse to unionize or vote for worker's rights?

There is a difference in interests at work between employees and employers. Even if someone doesn't own the business, if they have real hiring and firing power over other workers then they count as an employer to us.

I have two direct reports at work, so it seems like they consider me an employer, and I'm not eligible to join.