"The freedom to speak is only half of the freedom of speech. Compelling speech, under most circumstances, violates the First Amendment just as much as prohibiting speech does. For example, I can't force you to put a Ron Paul 2012 bumper sticker on your car, no matter how awesome that would be.
Due to federal labor laws, unions can obtain a monopoly power to negotiate on behalf of employees who don't necessarily want that particular union, or even any union, to do so. Because of these laws, it's actually illegal for employers to negotiate with these workers outside of their union. If these workers want any input toward negotiating their working conditions/wages/benefits/hours/etc., they have to do it through the designated union. This requires, at a minimum, paying union dues.
Even though unions are ostensibly voluntary organizations, and cannot compel employees to pay dues, this doesn't bear out in practice. First, as explained above, the federal labor laws actually make it illegal to negotiate with their employers outside of the union, so many feel obligated to join. Also, union backers have repeatedly subjected employees to all kinds of pressure to join, ranging from subtle persuasion to flat-out violence. Union membership is something less than voluntary for a lot of people. Things like card check legislation would exacerbate this problem.
Getting back to the free speech issue, money is fungible, and when union dues are inevitably used to support political messages and/or candidates with which some of the union members disagree, those members have effectively been forced to support a political message against their will. That is wrong.
For example, if the Kansas Bar Association required attorneys to pay membership dues in order to practice law within the state, and then spent those dues advocating for political causes abhorrent to some of its members, that's also wrong.
No one would argue that a school district could require teachers to display a Bush/Cheney bumper sticker on their cars. It would be an obvious First Amendment violation. But, if the union for those same teachers spent a bunch of their dues on television ads for Democratic candidates, it's really not much different.
Even unions recognize this, which is why many already maintain separate accounts to separate their political and non-political activities.
If that's what the bill is getting at, I'm fine with it. I hate fixing the unintended consequences of legislation with more legislation, but state legislatures only have so much they can do in the face of federal law and the manner in which the Supreme Court interprets the supremacy clause. It would be better to repeal the vast majority of labor laws that created this mess in the first place."
The very next day I had an almost identical question on the MBE.