In a hypothetical situation where you have two companies, lets call them Company A and Company B, who are in talks for an acquisition. If Company A is huge and is potentially an acquirer then Company B would be it's acquisition target. Company A is going to learn a lot about Company B to consider purchasing them. Company A could walk away at any time with all the salary information of Company B's employees.
Company B should have the right for protection in such a high risk situation. A non-solicit agreement that covers employees would prevent Company A from approaching, with the intention of hiring, Company B employees for a period of time that both firms agree upon.
Employees of Company B are more than welcome to approach Company A about jobs. Company B employees may even get a raise since Company A would know how much they are earning. However, Company A would want documented proof to show that the employee of Company B came to them. As they cannot approach the employees of Company B during the previously agreed upon period.
The system needs to protect employers like it protects employees. Not everyone runs a high margin billion dollar company.