I don't understand why you left a tip when, even as a foreigner, you knew that tipping isn't customary in Japan. When I go to a country where tipping is the norm, then I do it (much as I disagree with the system). When I go to a country where tipping isn't the thing, I don't tip and I cringe when I see people trying to alter the local culture with their own customs by trying to force tips on people. Please, just don't do it, it's offensive.