how exactly does it violate your privacy to be merely allowed (not forced) to discuss your salary?

Because you’re already allowed. Nobody comes knocking for an arrest when you tell your friend how much you make.

But…it seems people have been fired for disclosing how much they're paid? Some places, the employers have language in the contract banning it, and some of those some-places have courts that allow it.

Where are you being banned from discussing your salary? If I stop you from wearing shoes in my house I’m not making it illegal for you to wear shoes.