Like cryptography[1], rocketry has an inherent political character. If you disagree, then how would you feel about training a team of North Korean scientists in the type of long-range rocketry involved delivering a payload to a target as small as ISS?
I personally trust China with rocketry far far more than I trust North Korea, but "Do we trust this country with knowledge useful for killing large numbers of people?" is totally within the scope of a government to decide.
But haven't China proven (again) that they already have that technology? Unlike the DPRK (who don't seem too eager to get into orbit, either). Not working with China in space feels like a rehash of the cold war space race, except the US and the PRC are quite a lot friendlier, at least in terms of trade, and travel.
NK is indeed working on delivering payloads to orbit. Their latest mission was declared a success, although most watchers determined the orbit was so bad it would be coming down soon. I didn't follow the mission closely so I don't know what became of it.
I personally trust China with rocketry far far more than I trust North Korea, but "Do we trust this country with knowledge useful for killing large numbers of people?" is totally within the scope of a government to decide.
[1] http://web.cs.ucdavis.edu/~rogaway/papers/moral-fn.pdf