If a hiring manager wants to use this data to gouge employees then that's a problem with their ethics, not OI. The place where OI is incredibly useful is for making fair offers, especially for roles you're unfamiliar with. Maybe your company has 6 engineers and you're about to hire your first PM. What should you offer them? A market salary would be $X. You are not trying to figure out X so that you can make a lowball offer; you're trying to figure out X so that you don't offer 1.5X (which hurts your capacity to make future hires) or 0.5X (which might come off as stupid or insulting to the candidate). OI tells you what X is.
Data transparency is a good thing. It helps people avoid dumb mistakes. To be honest, I think it would be great if employees could have access to this data, too.