Not quite. It's an implicit right (whether fundamental or not), and the courts recognized freedom of association as being implicitly necessary to fully exercis

Not quite. It's an implicit right (whether fundamental or not), and the courts recognized freedom of association as being implicitly necessary to fully exercise the rights explicitly recognized in the first amendment. It's not freedom to join vs freedom to leave, but whether a contractual agreement can limit this right; can a contractual agreement exclude a party from associating with other parties. Exclusive labor contracts are one example (presuming the company's agreement to exclusivity was truly voluntarily, which is debatable), but what about other contracts? Given that every purchase is a contractual agreement, could Sony put a clause in the sales agreement that says the buyer can never buy another brand of TV? What if it was only while you owned and used the Sony TV? What if your Sony TV discovers another brand TV on your home network and bricks itself until you get rid of other TV? The right to a secret vote is equally implicit in the constitution, but we wouldn't hold a contract that forfeits that right to be valid, just as a contract requiring someone to vote a certain way would be illegal.