That is demonstrably false;
"Increasing the national seat belt use rate to 90 percent from the current 68 percent would prevent and estimated 5,536 fatalities, 132,670 injuries and save the nation $8.8 billion annually."
If you hold a libertarian viewpoint, that $8.8 billion counts as "harm", (I don't, so I don't consider that "harm", nice try though)
Those 5,536 fatalities and 132,670 injuries counts as lost productivity
And if the goal is to ensure the most good for the most people, how can you possibly effectively argue against an enforced seat belt law? What about the emotional grief of unbuckled auto accident victims? How do you measure that in a purely utilitarian model?