Burden them with new mountains of expensive paperwork, the costly staff to compile it, lawyers and accountants to review and consult on it. That's not nearly all. It would further require an expensive and expansive new taxpayer-funded state bureaucracy, complete with attorneys and auditors, to conduct oversight and compliance. Union members can do just that. Under federal law, those who choose to pay dues can already withold them from politics.
Voluntary support is allocated by majority vote.
Unions are already the most heavily regulated private organizations in this country which imposes the most repressive labor laws in the industrialized world. Attempts to impose additional micromanagement by government serve only to further weaken worker power to fight shrinking paychecks and dwindling opportunities. Taking the only organized voice of workers out of the political game produces legislatures committed to maintaining this plantation in the style to which its owners have become accustomed. Never sign nothing was a rule old gamblers lived by. It remains a good one today as adolescent Nevada fights growing up.