most likey a few reasons
1. Bill still more known to the world bigger ego boost for Korea
2. Hillary equals its a federal goverment offical high ranking...it carries different meanings and status that North Korea has not been welcome to, and is not diserving of for this.
3. as a private person, he has more flex and can work for the goverment...and not have the "goverment" do anything...Much like why we dont have people in N Korea,,,but go threw Sweaden I belive for contact...sure its realy the same thing as talking with them...but politicaly it is not.
4.as for what we gave them.....they get to say Bill CLinton meet with them ( boost them up a bit to meet a powerful figure) and they no doubt know that it could help them look better in the worlds eyes...even if just a bit...but just enough...and realy what do they lose? I doubt they got anything big, and if they did would not say anway...but fact is those 2 where not worth much in the grand world view of the battle of the US and Korea.....only to thre freinds and family...