It's Right to Work laws that tie the hands of companies. They prohibit companies from entering into agreements they might otherwise enter into.
Unions have nothing to do with assemblies of speech. The purpose of a union is to equalize the bargaining power of employees with that of employers. You or I or anyone is free to, for example, apply for a job at Microsoft, but we would be one individual negotiating for his sole income against a multi-billion dollar corporation, a negotiating disadvantage. If all the employees of Microsoft could organize, just as Microsoft itself is organized, then it would be two equally balanced entities negotiating a single contract, rather than thousands of very unequal entities negotiating thousands of contracts. What "organization" means is a way to overcome the collective action problem. This is inseparable from enforcing that all employees must be members of the union. In other words, this is the whole point of the union.
If you want a Right to Work law, I want a Right to Donate law. The website
Kickstarter is in gross violation of the Right to Donate. They allow people to set up projects that I can only donate to, if I pay Kickstarter a fee and so do all the other donors. I can only donate to these projects through Kickstarter - essentially by signing an enforced agreement with Kickstarter. That should be optional. We're just a group of people who want to peaceably assemble and give away money. Why can't we do that? Kickstarter should be illegal.