The Golden rule yes.
Employers are able to decided who to hire or not, just as employees have their right to choose who to work for or not.
Anything more or less infringes the rights of one side of the equation.
If you've ever heard the horror stories of union abuse in public contracting you might reconsider. They never served a useful purpose and still do not beyond union protectionism, the very reason they exist at all.
In a nutshell it boils down to the free choice of individuals - and whether employee or employer, unions prevent it. Not to mention the burying of merit, the foundation of free markets and the innovation that raises ALL our standards of living.
If you are a non-union tradesman, who moves to a non-right-to-work state - you will join the union or not work.
If our pathetic education system isn't enough to convince you of the folly of non-merit employment, I don't know what would.
I believe that you can see the same type of thing in the history of Russia and the Soviet Union. The autocracy and brutality of the Czars, replaced by the bureaucracy and brutality of the communists. Both sides wrong, but like a pendulum, swinging wildly from one side to the other. In the early days of unions, people had to make harsh choices as to whether they would work in unsafe conditions for very little money, or basically watch their family starve. Now, due to unions, people have to make harsh choices as to whether they can put up with working in a union environment, or do something else. I know exactly what you are talking about when you speak of the union shop and factory. I wouldn't work in one. Having read a fair amount of history, I do kind of understand where it originated from.
To say that pure capitalism would have solved it on it's own, well, I'm just not too sure of that. I'm a little suspicious of "pure" anything anymore. These type of things (unions, communism,etc.) weren't created in a vacuum. They were bad reactions to bad things. If we could travel back in time to see the circumstances of some of the workers back in those days, I believe you might temper your opinion on this a little bit. If the early capitalists hadn't been so greedy and heartless, some of this socialist crap that we live with wouldn't be around. I just want people to understand that there are consequences from having a heartless capitalistic mindset, and one of those consequences is socialism. A capitalist society underpinned by Christian values is the ideal system, in my opinion. For the most part, what I see in corporate America today is not that, and I just want to point out that there will always be consequences to that sort of thing. To say that capitalists had no responsibility in the creation of unions, is to ignore quite a bit of history. I'd like to see this "pendulum swing" find it's center one of these days. The ONLY way that will happen is if both sides learn from history.