They are firing missiles because they are manipulating the markets to make a few quick bucks. Nothing more, nothing less. I think this is actually known by all sides as the least damaging option for them.
Remember, >50% of USD are circulating across the world as reserve currency, and almost all commodities are traded in USD. China, Japan, and S Korea together hold a majority of this foreign USD. If we preemptively strike NK just because they can preemptively strike US, then the whole world economy can collapse.
BTW, Saddam and Qaddafi was taken out because he want to establish oil trade in gold pegged currency instead of USD. That's a death sentence right there.