f you were out enjoying the city last weekend, you might have stumbled, quite literally, across a new low in advertising. Big corporations like Nike and ABC were promoting themselves by pasting large plastic posters, known as street decals, on the sidewalks. This renegade marketing is offensive and illegal, and Nike wisely decided yesterday to remove its decals. There is a long tradition of graffiti artists, "guerrilla marketers" and fringe political groups painting, stenciling or otherwise inscribing their messages on the streets and sidewalks. This sort of defacement has always been illegal, but in the case of fly-by-night offenders it...