There's a difference between restricting your freedom and restricting your rights. When you enter an employment contract, you agree to give up some freedom in return for a salary etc. but it doesn't take away any of your rights. You still have the right to life, liberty etc. You still have the right to not turn up to work, though too much of that will likely cause them to terminate your employment. An employment contract that removes rights is typically not legal in sane countries (e.g. agreeing to become a slave).