Just off the cuff my thoughts are they are first choking off the life blood and giving specific consequences for bad behavior, and or know that if they did not sanction the 'Il' would take it all anyway and still use the same manipulative tactics with the general population (starvation etc) to control them. Lastly, it is a sickness and they want to contain it.
Think for a moment if the 'Il' spread his tactics to S. Korea and just ended up widening his territory of repression, as opposed to the opposite happening. At least right now the problem is isolated and has clearly defined borders (minus of course the behind the scenes support of the Chinese--but even this is a known factor). Thus is it easier to contain the threat.
Sometimes the unknown is worst than the known and this is sometimes why we see, in my opinion, small steps taken with countries such as NK unless they become a huge threat, versus just wiping out the leaders. Look at all the energy, time and money it is taking to rebuild Iraq. Can you imagine this happening in at least two to three other countries such as Iran and NK at the same time? It would leave the US over taxed and possibly unable to defend itself from even larger threats such as an attack from China/Russia etc. JMO. Food for thought. Thanks for the post.