Given the proximity now of the Bowery Hotel and, more recently, the Cooper Square Hotel... I'd say one too many clueless hotel guests plopped down with a cup of FroYo to take in some authentic NYC scenery. Woo, honey check out the mopeds! Sit on one while I take your picture!
Man, I wish I would have seen this!
[Update: In the comments, reader James provides more information: "The sign is actually for the guests of an illegal hotel next door at 73-75 E 3rd. I used to live there as the building 'management' was turning the building into a 'Vegan Hotel' despite 8 complaints into the department of buildings to my last count. Now they even have a blog... I can see how the Hells Angels could be upset. They are great neighbors btw, if you don't sit on their bench."]
Not too long ago, I saw a harmless-looking delivery guy start to chain up his bicycle on the light pole to the west of the HQ's front door. The delivery was for the building next door. From a doorway across the street, a man emerged and hollered with authority: "MOVE THAT BIKE." Somehow, the delivery guy missed the "No Parking Except Authorized Hells Angels" sign.