He is free to believe this. He is free to advocate that his state become a so called right to work state. It’s not the place of the Supreme Court to play the role of the legislature. Janus’ claims to having his free speech impinged are ridiculous. He can’t point to an aspect of free speech that he can’t now do but could if he didn’t help pay for his collectively bargained contract. His claims in this regard are ridiculous.
He’s already entitled to a refund of the portion of union dues (if he decided to join it) that go to political activities. This is the Beck decision in the late 80s.