] By appearing to disregard a series of ever-escalating
] actions by Pyongyang, the US may hope to alarm its allies
] and partners in the region to become more involved
] themselves in the North Korean problem. That would help
] bridge the gap between those who want the US to enter
] direct talks with the North - something the US wants to
] avoid - and the US view that Seoul, Beijing, and Tokyo
] must be part of any resolution.

A good analysis of U.S. Strategy in N.K.

Why US is reacting quietly to N. Korea's provocations | csmonitor.com