"What announcing the doctrine of "proportional" response does is lower the price of aggression. Why would we want to do that?"
The wizards of smart aren't announcing this to the North Koreans (or other enemies), they are announcing it to their peacenik friends, in hopes of comforting them that these wizards are not blood thirsty war mongers, as they have labeled their predecessors. They prefer to gamble with the lives of the masses, so long as they remain in good graces with their friends and supporters.
They are sick.