As I replied in another comment, the moral justification for unions is to prevent mistreatment of workers, not to transfer as much of the wealth created by a company to its employees as possible.
> prevent mistreatment of workers, not to transfer as much of the wealth created by a company to its employees as possible.
The wealth is created by the workers, not the company. And some would argue that keeping too large of a share of that wealth is, in fact, a mistreatment of workers.
[EDIT] that is, I don't follow why it's fine (presumably) for companies to seek to maximize returns, negotiating as a large block of people pooling their labor and capital, but not for workers to do that.
Similarly, just because some jobs produce more value doesn't mean the companies arent otherwise misbehaving.