Because it's bollocks.
No, actually, it's quite true. Just a few examples:
Dana Nuccitelli (of Skeptical Science - an AGW support blog) is actually employed at Tetra Tech (the big oil company).
Pew Charitable Trust's Center for Climate and Energy Solutions are principally funded by Royal Dutch Shell, HP, and Entergy Corp.
The World Wildlife Foundation also received a lot of funding from Royal Dutch Shell, and John Loudon (former Shell president) actually served as the WWF president for four years.
Standard Oil's charitable arm has given millions of dollars to Greenpeace, the Sierra Club, and others.
Four companies sponsor Stanford's Global Climate and Energy Project: Exxon General Electric Sluberger (Oil Field Services company) Toyota