Is it illegal to have cameras at work and not tell the employees?
No, it is not illegal to record employees in the workplace and not tell anyone. You are in a place of business and have no expectation of nor right to privacy. Places where you DO have a right to and an expectation of privacy are non-public spaces, like a restroom or changing area. There IS a question about whether audio can be recorded along with video and still be legal, but video only recordings, those made for identification purposes at ATM machines or to prosecute thieves, for example, are perfectly legal in any public place and common areas of any business in every state in the union.
