You have a simple choice: sign it or be terminated. Unless you have an employment contract for a definite term (e.g. for 2 years) which has not yet expired, you are an employee at will; that means that your employer may fire at any time, for any reason--including for "speaking up on issues" with the workplace, supervisors, or co-workers--without provoding prior notice. Thus, they can fire you if you do not sign. There is no right to free speech in the work place, and no right to bring up issues or try to improve the work place--even if you are 100% correct about everything, you do not have the right to do these things.