People voting for Bush can be blamed, but arguing “but you helped elect Bush” won’t get you very far with them. That’s exactly what they intended.
As for people who didn’t vote between Gore and Bush: that’s exactly whom I’m talking about. Whether they stayed home or marked Nader on the ballot, they are part of the same phenomenon. You’re right that the majority of that group stayed home, but the Nader campaign was part of it. In your metaphor: effective nonvoters are CO2, and Nader voters (in swing states) are the CO2 emitted by private cars. A minority, but still very much a part of the problem, and worth talking about.