those rules were written up by a company called "ePrize"
here they are:
https://nationalgeographicandbestbuy...cial_Rules.pdf
the cited part is towards the end of section 5.
I don't really think this is anything too crazy. They just want tight control over the images they publish. Think about it. If your image gets multinational attention through their publication, people recognize it as a national geographic photo. Now, if you still own copyright, you can do as you wish. But national geographic will forever be linked with that photo. You could easily imply that national geographic supports whatever cause lends itself to that image.
Also, now if somebody sees that image somewhere they think it shouldn't be (a background in a car ad, for example), national geographic will have a very hard time tracking it down. Is it stolen from them, or did you sell a license to somebody who could have sold the license further down?
I realize this is a little "out there", but you gotta cover your own ass
