Stealing from a restaurant is a criminal matter because you're taking (by force or concealing your actions) what doesn't belong to you. A restaurant not paying wages is a civil matter because they failed to fulfill their contractual obligations to pay you and are in debt. Arguing that taking money by force and not paying money are equivalent is comparing apples and oranges.
(It may be appropriate to criminalize failing to pay employees, but I'm not sure about the first and higher-order effects of doing so, or how to craft legal principles and laws with the right incentives, while avoiding special cases like the laundry list of exemptions to California's AB5.)