The problem arises when other people try to implement a compatible tool. Sure your JSON rules might not be ambiguous, but they are not standard either so every competitor aiming for compatibility will have to adhere to these specific rules.

This means that they need to use samatman-JSON and cannot really on JSON libraries for parsing. Or worse, if these tools become popular, we will see "samatman-rules" flags arising in JSON parsing libraries thereby harming the simplicity that made JSON what it is to today.

I think that your example illustrates exactly what makes Postel's law harmful. If you accept JSON, stick to the rules.