There's a difference between what's legal, what should be legal, and what you can successfully bargain for. If employee wages were being stolen, that's a matter for police and lawyers. A union exists mostly to help employees represent their side of the latter two cases, in a way that's as organised as a company.

The article describes a situation where employees were blocked from taking holiday time by management, and where they collectively bargained to fix that problem. That kind of thing is not the same thing as "the inability to get fired", and I think there's a much stronger case that it's a net positive for society.