"Agrees" is doing a lot of work in that sentence. Agreeing to $100/hour because you think it's fair and agreeing to $1/hour because you're starving may both qualify, but they're quite different.

That would be true if there weren't a power and information imbalance.

What would be the downside of all salaries in all corporations being pseudonymously public?

It would be prudent if you would dig a bit deeper into how unions came to be. Long story short, capitalism can easily create situations where you agree to be exploited to some degree, to avoid being exploited to another, worse, degree.