Of course they can. There are no states where all the jobs are union jobs. They can go work somewhere else that is non-union. They can also vote out their union leaders.
Bull. If a pipe fitter wants to get a job he's qualified for in a union state, he has to join the union. He doesn't have a choice. Saying he can work at Burger King if he doesn't want to join the union is Stalinist horse squeeze. According to that definition of "voluntary," paying protection money to Guido the local leg breaker is "voluntary" because you can close your business and move if you don't like it.
So why do I have to sell my stock, then? If I really like Apple's products and I think they are going to make a lot of money, why should I be forced to have Apple spend my money that I have contributed to the company in ways I think are politically inappropriate? Why should I have to miss out earning a boatload of money because you think I should sell my stock because I don't like Steve Jobs's politics? How is that any different than the pipe fitter who has to miss out earning money as a pipe fitter and instead has to work at Wal-Mart?
I'm not necessarily pro or anti-union. I'm just pointing out the hypocrisy of your argument.
If you think that union members should have the right to dictate to the union where their money should go politically, why do shareholders not have the same rights with their money in a corporation? Why should shareholders have less say over their money than union members? Why are shareholders less worthy than union members?
Oh, and your argument is wrong on another level. If your only choices in a 401k plan are mutual funds, the shareholder cannot sell his stock. He has to own it. If he is part of a defined benefit pension plan, he cannot sell his stock. He has to own it. And of course, not every shareholder can sell stock.