I'm in Canada, which is a different legal jurisdiction. A clause saying you can't discuss how much money you and your colleagues are getting is unenforceable here, and attempting to enforce it will only earn the employer large penalties after the labour board hearing and the consequent lawsuit.
Something about speech, assembly, and association rights. They are the underpinnings of labour unions... if you hate unions, you must necessarily hate civll liberties. WDYHASM? :D