Funny story. For several years before joining the US Navy I rode Sportsters. No helmet lock. I always wore a good full face helmet and just left it sitting on the rear view mirror when I went places. No one ever stole it.
Soon after joining the Navy I got rid of the Sportster, too unreliable and in any military organization there are consequences for being late, and bought the first of several BMW motorcycles. Old BMW "airhead" twins have a nice helmet hook that is accessed from under the lockable seat. When the seat is down the loop is closes off at the top. Very nice arrangement. But I guess a Harley thows off this menacing vibe and their riders are perceived to be, shall we say, disposed to be surly and good fighters, while the BMW fraternity are perceived to be a bunch of puss1es. Well, someone cut the helmet off the lock. Nice. The first bike I own with a helmet lock and I have my first helmet stolen. There is no gravity, the world just sucks.