The freeze is mostly ineffective for when you actually want it to work. From what I remember (even for the credit freezes) is that if you provide written consent to, say, a background check, then that overrides your freeze. So if you're applying for a job (basically the major instance where you'd want your salary information private) they're going to ask for your consent to do a background check and bingo they'll know how much money you make.

IMO this type of information should be illegal to sell or request.