Steve Clemons brings up a good point:
Iran and North Korea know that America's bark is loud but bite is probably pretty soft right now. And the Europeans are doing their best to take on a global strategic dilemma -- their very first -- without the U.S. in the lead.
The EU is in the beginning stages of superpower-dom, their ascension accelerating during a period of American decline. The Iran problem, which America has repeatedly failed to solve and generally made worse when they've tried, is the first dispute on which the EU can manifest their vision of a superpower that relies on diplomacy to defuse international threats. This is a proving ground for them, and they're going to do their damndest to succeed where we've failed, thus legitimizing their alternative vision of international relations. Considering how tricky a problem and serious a threat Iran is, that the EU's negotiators have something to prove is an unadulterated Good Thing.