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China is an emerging global power, and a rapidly growing market.  That's a major inducement to do business, and to ignore things which might get in the way of that.  Less cynically, engagement is more productive than isolation, in terms of cultivating progressive attitudes.

With North Korea, market power isn't what is keeping the US talking.  What NK has to bargain with is the possibility of destabilizing a region the US wants to keep stable, and posing a threat to countries that the US has assumed a responsibility to defend -- South Korea mainly, but also Japan.

In the short term, talking with NK keeps them from causing trouble in the region.  In the long term, it offers the possiblity that NK will gradually become more open and more economically healthy, which will probably also make it less likely to pose a human rights problem.  It's a much better strategy than criticizing them for their problems and shutting them out of the international community, which might make us feel better about ourselves in the short term, but will continue to do absolutely nothing in terms of effecting change in North Korea.