I guess because when I was coming up, the sign of the clenched fist was being used primarily by Marxist revolutionaries and race nationalists. It was not a sign of civil liberties http://en.wikipedia.org/wiki/Civil_liberties in the general sense, more a sign of unified struggle.

Adopting the "worker's revolution" symbolism also seems to downplay the strengths in many cases. Groups like Anonymous or the readership of HN are far from being mobs armed with farm implements, they're the techno-elite.

Personally, I think a better sign might be the extended middle finger. :-)