It happens all the time. You yourself have said that it is the "threat" of possible strike by the unions that "compels" companies to do the "right thing". You have even said that most companies if given a choice would NOT do the right thing if it were not for the existence of unions.
Most contracts have a "no strike" policy in them. In other words, no wildcat strikes at the drop of the hat. I do not recall saying the threat of a strike will compell the company to do the right thing. The chance of a union coming in will help a company make the right choices. Organized companies chose to have the workforce union. If you treat your employees right, give them fair wages and benefits, you will not have to worry about your employees getting representation.
Any company that has a lick of sense will have strike insurance so if there is a strike, they will be compensated for the income losses due to the strike.
As a foreman in a union shop, I see the supervisors that have issues with their employees. Guess what, I don't have issues. I treat my guys with dignity and respect. They are all grown men and if treated so, will act like grown men. Treat them like children, and they wil act like children.
Therefore it is clear that in the general case unionized companies are "compelled" to sign union contracts due to the "threats" that union members can carry out if they do not sign. The common threats are strikes, work stoppages/slow downs, sanctions, and fines.
Welcome to life. We don't speed, not only because of safety reasons, but the threat of a speeding ticket. We don't shoplift, not only because it is wrong, but also becuase we could get arrested and get sent to jail.
I guess your definition of duress and mine are different. I see that as a "You must sign this or we are going to beat you up", and yours is "If I don't sign it, maybe they may threaten to beat me up and I am scared so I will do something detrimental to my company and hope for the best".
I don't buy that. 99.9%of workers out there ask for a fair wage with fair benefits. As the saying goes, pay me now, or pay me later. I can make $10.00/hr pay with another $10.00/hr put away for my retirement, or you could just pay me $20.00/hr to work for you. The difference is that by paying me 10 now and 10 later makes the books look better. If you are lucky, you can squander that money saved for retirement for your employees and have the blind sheeple say, "Those greedy union guys, they didn't care for the company".
Tom