This is impossible to enforce, and secondly I believe it would be absolutely unfair. Being fired because they cost the company too much money, and are unwilling to accept a smaller pay check, should be a valid reason to fire them.
It would be impossible enforce because there are many ways around it. For example not enforcing company computer policy, and then using it as a means of firing them. I was once told a story about an employer telling their employees they need to lower the contract hours temporarily because there was no work. All of the employees were fine, as it would ensure their jobs are safe, except for 3. Those 3 employees were fired the next day for violation of the company internet policy for improper use of email on 3 occasions - a legitimate reason of firing.
As I said, impossible to enforce.