Can employer of consultant restrict talking about wages to client?

I work for a consulting firm, as a consultant I have been told we cannot discuss wages with our managers on the client side, I rarely have interactions with consulting firm itself, all they really do is give me the paycheck, can they really have a clause in the contract that states I cannot discuss wages with my direct supervisor from the client side? These are the people that would be hiring me directly if the make me an employee in the future anyways.