I expect it's something along the lines of "Everyone on HN is already aware about SOPA/PIPA, so what would be the point?"
The same could be said of Reddit, but it is a symbolic gesture and really encourages you to call up your representative.
I haven't seen anything about SOPA/PIPA on Reddit, apart from the warning that they were going to be down protesting against it. It's just not come up on the subreddits I read. I imagine it hasn't shown up on most non-politics non-internet subreddits.