Because democracy is about society and not free will (it's about deciding what's better for majority) and yes, I do belive that limiting him that choice(not really him but his employer) is beneficial for society. It similar case to giving free access to alcohol, hard drugs, junk food etc. Someone has to pay this cost of nonoptimal choices.
EDIT: example: you don't provide living wage so some/more ppl will chose to steal. This might destroy their family, so social worker is needed, you also need more police, judges, prisons etc. All of them are employed, happily contributing to GDP. Success! Is it better world? I don't think so. It's much cheaper if with offering living wage. (as global optimisation, company owners and individuals do local optimisations)
EDIT: example: you don't provide living wage so some/more ppl will chose to steal. This might destroy their family, so social worker is needed, you also need more police, judges, prisons etc. All of them are employed, happily contributing to GDP. Success! Is it better world? I don't think so. It's much cheaper if with offering living wage. (as global optimisation, company owners and individuals do local optimisations)