Working is purely voluntary too, you still can't tell your employees that they need to vote in accordance with company policy. Just leave the church? Well for those who have faith in that path, that's holding a vote vs. damnation and you don't see what's wrong? This is the biggest church in the world attempting to effect democracies around the world... Your answer is that they can just leave?. I think it's much better to insist that you can't hold something over another to affect their vote. There are already laws in that area, this really should fit.
I don't know whether a company can force a person to vote a certain way or not. Maybe there is something in the Voting Rights Act and/or some other federal or state law that addresses employer/employee/voting issues. Never thought about it.
But regarding churches, I said this is "dumb." I don't think a church should tell its members how to vote. What you raised was the issue of illegality. I don't see how the government can regulate demands a church places on its members.
There are many things churches regulate, including the private sexual conduct of its members. For example, in Hawaii an employer cannot fire someone because they are homosexual. But a church can remove a member because of homosexuality.