Earlier quoted context omitted.
First, I apologize for the reddit style. I deserve the down votes, which I will happily pay. Using a position of power to force employees to divulge personal information really pisses me off.
There are three companion posts to this one that you might want to read because they'll fill in some assumptions you're making. Primarily, for existing employees the program was opt-in, such that "[n]ot joining would mean that others won’t know about that person’s salary and that person wouldn’t have access to the salary list."
There are two problems with that:
1. Even if employees get to opt out, others still know they opted out. And that can create mistrust on both sides. "Why did Bob opt out? What does he have to hide? Is he making more than everyone else? Is this transparent salaries thing just a way to keep the lower paid people thinking they're actually average?"
2. By excluding those people from the salary meetings, the company basically creates an "in club", the people that know more than everyone else. The opt-out people are now no longer just opting-out, they are literally being kept in the dark about things being shared with others. And like in scenario 1, that will breed mistrust.