Some people want corporations to be socially responsible. That is, they want corporations to have to be responsible not just to the owners of the business, but to "stakeholders". A stakeholder is anybody too cheap to buy a portion of the business, but who wants to be able to benefit from, and participate in, the operation of the business.
A union is organized as a corporation, no?
Thus, I claim that I am a stakeholder of the Communications Workers of America (just to pick on one of them), and the next time they vote on a union contract, I get to vote on it too.