I think even internal criticism of companies should be considered free speech and protected as such. That doesn't mean that it should be released to the public, but employees should have the freedom to release the information if they decide so, just like whistleblowers.
> Why is it our business to see who makes what?
Of course it is your business (if you are an employee). Employee wages are determined by supply and demand, therefore, they depend on other's people wages. That aside though, any exposition of the way other employers treat other employees in similar position (such as discrimination) also should be your business, because it may affect behavior of your employer as well.
These are the reasons why we have free speech, to prevent people doing wrong things in secrecy, as to avoid collective moral correction to take place.