> I don't understand why so many staunch advocates for free markets easily opt for the former.

It's pretty obvious to me: this information asymmetry between employers and employees benefits employers, letting them lowball some hires. More information would make hiring more fair, but also hurt the bottom line, so they don't want it.

Companies generally don't hire at all if it's going to hurt the bottom line.

There might be some exceptions around high profile positions (i.e. CEO) if there's an expectation that media attention and other factors might turn that loss into a profitable long-term investment, but it certainly isn't going to apply to your run-of-the-mill blue collar, middle manager or software developer position.

What hurts the bottom line is the company paying you the maximum that they would be willing to, rather than as minimum for which you'd still accept. Either way they come out ahead, they just come out more ahead in the latter case.