You know, there is no general law requiring shoes in stores. There may be local ordinances.
Back in the 60s, in Florida, hippies used to walk into restaurants right off the beach in nothing but a swimsuit or shorts. The establishments had the right to refuse service to anyone for any reason back then. Rather than have managers and wait staff tell them they weren't welcome, one entrepreneur printed up signs indicating "No shoes - no shirt - no servce." It caught on quickly, and soon you could see them everywhere. Eventually, it was just assumed there was some health code prohibiting it.
Actually, there is no law in most localities requiring any clothing whatsoever at a restaurant. Nude dinner parties have become a minor "thing" in major metro areas. ("The Füde Dinner Experience") The larger nudist resorts I've been to have a restaurant open on weekends and a bar on weekend evenings. Only a towel to sit on was required. Some states or localities will prohibit alcohol from being served, using laws against liquor at adult entertainment venues. But fortunately, California doesn't consider nudie resorts to be adult entertainment.