It's WTFPL-2 not public-domain. https://github.com/scrooloose/nerdcommenter/blob/master/plugin/NERD_commenter.vim#L7-L11
Thank you for the report. Indeed this license is different from public-domain: "Everyone is permitted to copy and distribute verbatim or modified copies of this license document, and changing it is allowed as long as the name is changed." The best solution for users and distributions would be an approved open source license: http://opensource.org/licenses/alphabetical Bug report upstream: https://github.com/scrooloose/nerdcommenter/issues/217
If you think about it this way, then $ eix --only-names --license "WTFPL-2" | wc -l gives me 28, from which 3 are from app-vim category.
Fixed in the tree.
