Unions represent the employees as a group vis a vis the employer....that's what unions are all about. It's called "collective bargaining."  Union contracts are "closed shop", meaning you are either a member or you pay "fair share" to the Union to pay for the negotiations which obtain the conditions under which you work.  The conditions agreed upon during "collective bargaining" are hardly arbitrary.  The Union is run by the workers and the negotiating team is selected by the workers through the union.  Unions have legal status as the "contracting" agent for the workers. 

 Explain to me why someone should be able to get a job at a "union" business, obtain the salary and benefits the Union status has achieved through negotiations and NOT contribute their "fair share" to that process. That's "freeloading."  

 BTW: Unions have long been in the forefront in the battle to attain equal pay for women.  Unions have achieved and protected that "equal pay for equal work" for men and women.