I would venture to say it's at the discretion but, going by definition and how most shore passengers might be doing (short overnight stays), they are staying within the vicinity of Narita City, Chiba Prefecture (or whatever city or prefecture the airport maybe at). Kanagawa Prefecture, Yokohama on the other hand is some distance away and I could see how it would be out of range for someone on a shore pass. Of course there's no real way to track people once they get passed Immigration, unless they somehow got into trouble then it's found they are outside the "vicinity".