The cartoon shown below gets posted to FaceBook by people arguing for policies to reduce global warming. The implicit assumption is that all of the things they want to do for that purpose are good things that they would be in favor of even if warming was not a problem. It apparently does not occur to the people who post the cartoon that one implication of their posting it is that they have a reason to believe in, and preach, the threat of catastrophic global warming—whether or not it's true.
I expect that most of the people who post the cartoon, or approve of it, would see the point in a commercial context. They realize that the fact that someone is trying to sell you a car is a good reason to be skeptical of his account of its condition. Most would also recognize it in the political context, providing it was not their politics in question—many of them, after all, believe that criticism of CAGW is largely fueled by the self-interest of oil companies.
But it apparently does not occur to them that, for someone not persuaded of their policies, the same argument applies to them, that, from the standpoint of the people they want to convince, the cartoon is a reason to be more skeptical of their views, not less.