Actually no, The Law applies to Winter, but custom, such as guest right has application across the entire supernatural world. When Harry broke guest right in Grave Peril it caused a war, and he had to work damn hard in Peace Talks to to avoid doing exactly the same thing again.
Vassallage is recognised by custom across the supernatural world, Marcone recognised Tripper as his Vassal albeit reluctantly, Trippers attack on Winter (the company car) mean’t Marcone was responsible in the eyes of the supernatural world. By custom he was responsible, if he ignored it it would have entailed a severe loss of face and made things difficult for his vassals in their relations with the supernatural.
Custom plugs into the Accords, in parts no doubt codified. Same with the Code Duello, custom was eventually codified.
We see it in certain areas of modern law, for example “trade practices” is a term used in legislation, but the practices vary from trade to trade, due to custom and norms built up in those trades.