Most don't. They DO however, adjust prices to maintain similar profit margins. If cost of labor goes up, cost of goods must go up as well. Additionally, non-minimum wage jobs will like also demand more money, because their skilled pay is in part based against the unskilled pay of minimum wage jobs. Or, a third possibility, is that minimum wage businesses simply demand more skills for the same job (or they'll modernize). McDonalds, for example, would probably want someone with a college degree or maintanence experience to work on the equipment for $13 an hour. So your unskilled labor will be unskilled homless/jobless.
In short, fuck a mandated wage.