A person being robbed at gun point is being coerced. He is having a choice forced upon him by another person. Society, being composed of individuals all of whom do not want to be coerced has an interest in preventing coercion.
This is not true of having to work. The biological necessity of your body requiring food is not forced onto you by anybody. So nobody can be held responsible for it.
A prostitute being given the option to sell her body for money actually makes her life better, because depriving her of having to choose does not improve her life, while depriving a man of having to choose between being shot and giving up his wallet improves his life greatly.