I'm not a big capitalist myself, but:
The “work” of an employee is their product they sell to the company they work for. That's what the worker is to them. A product.
And in capitalism, you should shop around and know the prices of the product you're selling. And the prices in your area (office).
But, they don't want people to share their wage information.
But you can't negotiate well if you don't know your local prices. And that's for any product in any business.. apples or labor.
If you're selling apples, but they say you can't look at what all the other people selling apples are selling them for.. that's quite a disadvantage.
In capitalism, you can gather your buying power to get better prices, like Walmart using all their stores to negotiate a lower price.
But, unions are the same idea, gathering their product, labor, together to get a better price for the worker.
They want capitalism for them, but not for the worker. They want to close and control that market for labor as much as they can.
This is just another reason why the system is fundamentally unfair.