Said John Kerry on "Meet the Press" today, when David Gregory asked him: "This is a sarin gas attack, perpetrated by the Assad regime, this is a slam-dunk case that he did it?"
The term "slam-dunk" figured large in the run-up to the Iraq War. Hence the resistance. It should connote certainty, but the meaning got flipped. To use it now is to seem to say: How do we know you're not conning us? And I'm going to assume David Gregory meant the insinuation, because everyone's been making the comparison to the selling of the Iraq War.