Before becoming somewhat of an American pastime in the late 60s the Hell’s Angels were well established as the kind of cliche biker group that you can often see romanticised in shows like 'Sons of Anarchy'. They ran drugs and weapons, they were a terrifying presence whenever they blazed through town and yet they were utterly fascinating to many people reading about them in the papers. Even though they were considered so interesting they never embraced this popularity and refused essentially every attempt for journalists to approach them; that is until Thompson came along.
They seemed to see something in him that reflected themselves, perhaps due to his ability to brave their kind of hedonistic lifestyle or his love of all things with two wheels and an engine, and accepted him into their fold. Any time another member would kick up a fuss about him being a journalist two more would defend him, so he quickly became someone who was often seen cruising (is that the term you use? Rode with them? Galavanted? I don't know, I'm not cool enough to go near a motorbike so I don't know) with the MC. He went along with them for rides, for bar crawls and even for their most hallowed of events like camp outs that saw them consuming more drugs than fat Boy Scouts cooking S'mores.
Eventually they started expecting more and more from him, like being able to break into his flat whenever they pleased, so he soon made his departure from their little club. Smart move, especially as he did this before they got entangled with the turbulence of American politics at the end of the decade.