DontBeStupid
Look it up!
National Labor Relations Act of 1935, visited and upheld by the USSC in Texas & N.O.R. Co. v. Brotherhood of Railway Clerks.
Anything else I can help you with?
If you want a more recent case, I guess we could use Citizens United, which found that a Union is a person, and therefor has unlimited Free Speech per the First Amendment. This would naturally mean all First Amendment rights are protected for a Union, which would mean right to assembly.
They have all the speech they like (as the rest of us do)...BUT they do NOT have a right to foist thier beliefs on the rest of us since THEY are a clear minority of the working population...
Get it?
Obviously. And no one has said they can.