I would guess that the doorman just can't necessarily tell the difference between a lot of the dirtier bike messengers and the crumbbums and gutterpunks. But which building do you work in? i mean in my experience, i see a lot of messengers just locking the rear wheel to the frame and on some occasions outside of my building, I've seen them completely unlocked! I think thats asking for it but I'm not one to teach the lesson.