You are hiring up an engineering team and hope to hire up a team of 10 engineers paying 100k/yr each. So far you've hired 9 people at 100k and are working on filling that last slot. You find someone perfect but he has a competing job offer and is asking for 110k/yr instead of just 100. He's no better than the other 9 people you have already hired.
In a situation with relatively secret salaries it might make sense to go ahead and pay the 110k. It's only a 1% increase in your yearly budget which probably isn't a deal breaker.
But in a situation with relatively public salaries you're in a bad spot. You can either not hire him thus prolonging your search, or you can hire him and face potential moral/teamwork problems. Maybe you can hire him and bump everyone else's salaries up to 110k but a 10% budget increase might not be feasible. So the company is a bit screwed in this situation. And to the extent that the companies success is also the success of individual employees the individual employees might be a little screwed too.
I've made this scenario simpler than what reality would generally look like but the same principal holds.
In nearly every situation in which I've had insight into engineer's compensation I know for a fact that fairness has always been a top priority. But it's worthwhile to see that it sometimes can't be the only priority and to understand how the salary taboo fits into this.