I think the fear is that it will create animosity by making the information so accessible. On some level, the junior-level dev knows he's not making as much as the guy with the PhD in an esoteric mission critical topic, but he has a kind of plausible deniability that lets him avoid thinking about it. If everyone's making 80k and everyone knows that Jane The Expert is making 300k, there's going to be friction. "Hey Jane, why don't you pay for the dinner."