Outlaw biker culture (that is, the "1%ers", the people with the three part patch) is about defining ones self against the normal. About shocking the car-driving, peaceful norm by your actions, and so bringing you into a close fraternal society. One of the ways to do that - especially in the late 40s / early 50s when Sonny Barger et al formed the Hells Angels in California - is to use the imagery of the recent foe, and German imagery is more obvious than Japanese, and there was less racism against Germans than Japenese, making it less difficult to wear German insignia.
I don't think you can have the same biker culture and have the wearing of Allies kit, even Soviet - when it was starting they were still friendlyish.