In effect, Buffet is now making a demand for green companies for copy-cat investors to invest in.
http://www.amazon.com/The-Essays-Warren-Buffett-Corporate/dp...
He is incredibly passionate about capitalism. Especially smart capitalists responding to pricing signals intelligently. And the symbiosis of good management and good investors to keep companies efficient, take smart risks, and earn consistent, healthy returns.
Making huge, counter-intuitive investments is nothing new for Buffett. (Railroads anyone?) I imagine he is doing this for the reason he usually does: he thinks he is getting a huge amount of value for a low price.
He may think that it would not have done that without his investment -- so it could be both, but Buffet promises investors that cash will be deployed more efficiently than you could by just buying an S&P index fund.
I don't think this isn't an investment, but I think there were motivations beyond purely investment; and having heard him speak and read what he has written, I am taking a guess that he was motivated out of an idea to influence good (while still making a sound investment).