There is an article at Reason magazine (http://www.reason.com/news/show/33169.html) from 2005 that details the difference in style between the fight against seatbelt laws, and the fight against helmet laws.

In particular it also offers some historical background on the origin of seat belt laws that suggest their enactment had more to do with the will of the auto makers (to avoid having airbag laws):

"In contrast to the motorcycle industry's stance on helmets, automakers played an early and conspicuous role in the debate over seat belt laws, which they began pushing in 1983 as an alternative to the air bags the federal government was threatening to require. That strategy took on a new urgency in 1984, when Secretary of Transportation Elizabeth Dole said the federal air bag mandate would begin to take effect in 1986 but would be lifted if seat belt laws covering at least two-thirds of the U.S. population had been passed by April 1989. (As with the drinking age, which every state has raised to 21 because of a 1984 law that threatened to cut off highway funding if they didn't, the supposedly federalist Reagan administration did not hesitate to impose its preferred traffic safety policies on the states through indirect means.) The auto industry set up a lobby group, Traffic Safety Now, and invested some $100 million to push seat belt laws in the hope of meeting Dole's deadline."