Why do liberals seem to hang out a lot on the streets?

And by this I mean the "volunteers" (paid or otherwise) who hang out on busy street corners trying to sign passers-by up for Greenpeace, PIRG, or whatever liberal cause there may be out there. One favorite corner is near the Borders on Friendship Heights which I happen to frequent frequently. But lately it has gotten so bad that I've either avoided going there (good because it stops me from spending) or I've taken the long way around to avoid them (good because it helps me exercise). I wonder why the NRA or pro-lifers don't seem to tack the same way.