The same reason why we see sugar with label "carbon free" and water with label "no GMO". Marketing. Yes, it gets old with time and they have to look for the next gimmick. But if that brings them some income in the meantime, that's what they do.
I'm not thinking of anything, it's an actual registered trademark. Of course, they meant "carbon neutral", as in "carbon dioxide emissions neutral", but that's not what it said on the label. It said "carbon free". Yes, I studies chemistry in school and know that carbon-free sugar is water :) But people who are the target audience of this marketing gimmick don't really seem to be bothered with it.
