law - administration
In the United States in 2009 there seems to be the following regulation regarding the question of landing at a closed airport.
(a) The pilot in command of an aircraft is directly responsible for,
and is the final authority as to, the operation of that aircraft.
(b) In an in-flight emergency requiring immediate action, the pilot in
command may deviate from any rule of this part to the extent required
to meet that emergency.
(c) Each pilot in command who deviates from a rule under paragraph (b)
of this section shall, upon the request of the Administrator, send a
written report of that deviation to the Administrator.