I find it just disgusting they do this. How would jw's feel if Christians went calling on them on the day of their reject Jesus Memorial? How about if a group decided to hold a prayer vigil in or near the parking lot complete with signs and candles hoping to 'save' the jw's?
I don't think they do it to truly preach to anybody, they only do it so they have good stories to tell at the kh and to feel even more self righteous (as if that's possible) than they arleady do.