It the case of labor unions it is due to the right of their members to freely enter into any contracts they choose without undue government interference. The members of the union decided it was in their best interest to negotiate their contract as a group. Union contracts or "collective bargaining" are perfectly fine as long as the negotiations are free and fair. They must operate within the confines of the US Constitution.
Where they go beyond the American Idea is using threat of violence against anyone who attempts to enter into their own free and fair negotiations with these same industries. They often violently injure anyone who attempts to work for a company who isn't a member of the union. They also use graft and corrupt practices to enact unconstitutional legislation requiring work in certain industries be the sole right of those in their union. This is unconstitutional and un-American. They are free to collectively bargain but they have no right interfering with my same right to choose not to join the union.
They also have a history of threatening and or damaging the property of any company which chooses to hire non-union labor. I once personally witnessed a UAW instigator strike a young woman in the temple with a tire iron as she drove off the company property through a picket line. She was a single mother of 3 who had recently escaped an abusive marriage. To get buy she was willing to work for what to her was VERY good wages placing resistors into the holes on circuit boards. It was a Godsend to her as it provided her the prospect to escape a life of deprivation and abuse. A union instigator stepped forward and pummeled her in the temple resulting in permanent brain damage. It eventually went to court to decide what damages should be paid to raise the 3 young girls orphaned as a result.