Good point! Depends on if the workers to vote to unionize and if so how big the vote was. If the unionize vote barely passed then those who voted against the union probably don’t have to fear retribution or feel forced to join since their number is large!
If the vote to unionize overwhelmingly passed then those that oppose the union would be wise to tread carefully or even look to work elsewhere! Like you said they can’t lock you out for not being in the union doesn’t protect you from union thuggery either!
And there's the real problem. Unions are government sanctioned criminal organizations, carrying out extortion by threat of deadly force.