Because the punishment is part of the input that determines what you will do.
In fact, if free will were absolute, punishment wouldn't make any sense because it wouldn't have any effect on your will.
In fact, if free will were absolute, punishment wouldn't make any sense because it wouldn't have any effect on your will.
If you have absolute free will you are free to disregard or consider inputs.
What is being claimed by a deterministic world model is that the output (behavior and internal state change) of a human is a pure function of its current state and inputs. Then we try to give inputs that will lead to desired outputs.
The non-compatibilist view of free will is that it is not a pure function, namely that there is a third independent factor, the “free will”, that influences the behavior (and possibly the internal state). If that is the case, there may never be a way to choose inputs that lead to the desired outputs, because the free will could simply void their effect.
If I tell someone not to do something again, then that is an input to their future states whether or not my decision to tell them that was freely chosen or not.
If you go into that situation with the belief that not having free will means that what you do does not matter, and your action as a result is to not tell them, then that will affect their future states too. And so whether or not you have a real, free choice, it is beneficial to act as if free will exist even if you see it as an illusion.
I strongly believe we have no free will. I still get up and work, and try to do as best I can. I believe those choices are not free, but they feel like choices, and they impact my life, so I am happy I act as if they are free.
And so I'll still talk about making choices and trying to do things because of that illusion even though I believe it's all a chain of cause and effect.