Why shouldn't it be? There are many reasons but the big one to me is the ability to bargain as a group allowing for greater leverage where without it you have none.
Only about 10% of the private sector is unionized. When I was going to college I worked in a union shop about 30 hrs a week. I had to pay union due's out of my pay check but got zero union representation or perks. But because I worked there I had to pay dues. I will never work for a union again. The union leadership are a bunch of greedy bloodsuckers.
Maybe the public sector unions in WI should be asking why the union leader is making 480K/yr. Public sector unions are a scheme to funnel money back into the democrat party, no being partisan simply stating fact, check union contributions for yourself. There is no reason for public sector unions, they exists on tax payer money, no wealth is created.
But you want to talk about freedom, there is no freedom on choice of joining a union, when working at a union shop.