Why cooperative approaches may stand up to legal review more easily in unionized plants than in non-unionized environments?"
This is a great question. There is a good reason why unions have more legal sway and clout. The reason for this is due to the fact that unions are powerful. They have the ability to speak and act collectively. So, this power gives them the ability to bargain, to challenge the law, and so forth.
To put it another way, it is wrong to break the rights of an individual. However, you might get away with it. To break the rights of a whole group is another story and you will not get away with it. When you look at it from this perspective, you can see why unions have a edge over non-unions in any setting or company. Finally, this is also way many companies do not want people to unionize.