The general idea is to prohibit closed shops, on the grounds that everyone has the right to unrestricted employment; that closed shops prevent those who will not join unions from working, and therefore infringe on their right to work. There is a fine but significant distinction between the right of employment itself and merely the right of free access to employment, but I found the affirmation of a "right to work," in those terms, rather ironic, all things considered. And, to be clear, putting it solely in terms of unions does not make the above distinction (which is in many ways the problem with right to work states.)