Whose laws should I obey in no man's land [duplicate]

You are most definitely not in no-mans land in the international departures terminal. You are on the sovereign territory of the country until your foreign-registered aircraft's wheels leave the ground. Border control is purely for immigration purposes, and it only exists in countries that have active outbound controls (Canada, for example, does not).

The only law that really changes after border controls is that you have departed as far as immigration is concerned. If you have to leave by midnight, and you cross the line by midnight, they are happy. If your flight actually departs at 1am (or is cancelled) you don't get busted for overstaying.

Lets turn your situation around and look at the US pre-clearance process in Shannon, Ireland. Once you have passed US border controls you are administratively in the USA, and your flight can land at a domestic-only airport and not only a port of entry. But you are still in Ireland. Do something legal in the USA but not legal in Ireland and you will be arrested by the Irish police; the Americans will have nothing to say about it.

If you really want to be in no-mans land there is a rather bleak segment of desert between Egypt and the Sudan that is claimed by neither country. There are also parts of Antarctica under no jurisdiction.