Because private company's shouldn't be handed money from the taxpayer to pay their wage bill, even you must see that is just wrong, not to mention how many of those private company's pay a little higher than the minimum wage but would not have to pay those on the job guarantee the going rate, that is exploitation.
Would you also disagree with tax credits - the state agreeing to top up salaries provides a system whereby companies can employ people on a low wage while knowing that that low wage is feasible because the govt will boost it artificially to something more livable and in doing so also provides an indirect subsidy to the company.