alt Hacker NewsThere'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).
In Germany your employment contract can have Vertragsstrafen, ie the contract can say that if you do something you need to pay a penalty even above and beyond damages.
But in any case, employment contracts are weirdly restricted in many places, exactly because lawmakers think workers are metaphorical babies.
You can have a look at contracts between equals to see that being able to restrict yourself is important.