They believe what they believe and everyone else had best shut up. They are our lords and masters and know exactly what is best for us. Welcome the politics of the Left. They will decide what you can say (under the auspices of hate speech, or denying debate like this) and so control eventually what you can think. What they want you to think will put more control of your life in their hands.
The letter is of a piece with what has become a campaign of intimidation against any global warming dissent. Not only is everyone supposed to concede that the planet has been warming--as it has--but we are all supposed to salute and agree that human beings are the definitive cause, that the magnitude of the warming will be disastrous and its effects catastrophic, that such problems as AIDS and poverty are less urgent, and that economic planners must therefore impose vast new regulatory burdens on everyone around the world. Exxon is being targeted in this letter and other ways because it is one of the few companies that still thinks some debate on these questions is valuable.What is the "or else" part of the warning? Well they weren't too naive; they just compared what would happen to Exxon, with what they did to the tobacco industry.
"Say hello to the new boss."