By Law non union members recieve the same benefits as union members. But they can't vote in elections. They get the negotiated raises and benefits. They are represented by the union in labor disputes. So it is fair that they pay for the union services and the expense extended on their behalf. But the argument on supporting union political or other non-service expenses is where the Supreme Court drew the line. Paying those expenses should be voluntary as supporting political positions against your beliefs involuntarily is a violation of free speech.