I read a similar thread over on another site.
The general consensus there is that a lot of it is tied into memorabilia and "war booty" from WWII, specifically Nazi stuff. A lot of returning GIs had "collected souvieners" during the war to include insignias, decorations, flags, arm bands, pistols, hats, uniforms, and of course helmets. When they returned the US Army started selling the surplus HD motorcycles for $25.00 and the former GIs soon found inexpensive and semi-reliable transportation. As they slowly formed "clubs" they often "decorated" themselves with stuff they brought home. Soon Nazi helmets, SS emblems and Deaths Heads were adorning jackets and vest. Most of society turned a blind eye to this remaining small percentage and just saw them as soldiers who once faced death continuing to push their lives and lifestyle to the limits. But overtime this small percentage was soon thrown into the rest of the "1%" and thus the bad image of the biker gangs were formed. Regardless, the enduring image of the deaths head skull and german helmet still lives on in a milder form of accessories and fashion wear offered by your local bike shop.
But I could be wrong, after all I read it on the internet.