Salary info is a very small piece of the information needed to evaluate the fairness.

The article details quite extensively how difficult it is to communicate the totality of information in all but a few circumstances. If everyone could see every piece of relevant info then I'd agree.

One example: my value to the company may be for something you see as worthless. But you don't have the full strategic roadmap of the company's leadership in mind when you devalue my contribution. And the company may have valid reasons for not making the strategy widely known in the company. If you learned that I got a big raise for my success in that area, you'd feel slighted. The problem there isn't lack of info re: salaries, it's the impossibility of every employee having every bit of info needed to make the assessment.