One of the roles of diplomats is to familiarize themselves with the foreign policy attitudes of opposition party leaders. Congressmen have a 1st amendment right to discuss anything with anyone. Obviously the Dems were only spelling out their attitudes, not negotiating for the government. The US does this all around the world, so does every other government.
Yeah, they weren't "negotiating" on behalf of the US, they were stating what they would do IF they were elected.