"isn't it a fundamental right to join whatever organisation you wish, not just the ones the government likes?" I agree with you on this, but unions gain power

"isn't it a fundamental right to join whatever organisation you wish, not just the ones the government likes?" I agree with you on this, but unions gain power from pro-union laws that enforce things like not allowing hiring non-union workers among other things, so it isn't as simple as you state. Without the legal backing, just joining an organization doesn't have much effect. This changes it from a freedom issue to legal policy issue.