Saggy pants just look really terrible.  Everyone (except the people that sag their pants) know this.  The state of Massachusetts knows this as well and has passed legislation making it illegal to sag your pants in public.  Seems like the penalties are pretty steep, but that's fine.  Don't sag your pants and look like a dick and you won't have to worry about it.  Massachusetts has even put together a PSA about pants-sagging!