Employers and employees should be allowed to talk about their political affiliation and the reasons behind why they do.
I think that this has basically the same problem as sexual harassment, frankly. You can't pretend that it's an equal relationship, because it isn't. When we start doing something about wage-slavery, I won't worry about the capacity of employers to apply pressure, but we seem to be a ways from that.
If I find out that a person was explicitly fired for their decision in the voting booth then I'll have a problem.
Is this really your standard? Because, again, there's a reason we use more stringent expectations to identify racism and sexism than "explicitly fired for it."