Not very smart...
they should have spent some effort to include what happens to the human corpses and their effect, or least make up a cause like all humans are abducted by the aliens...
just as silly as "sign"... LOL
That's not really the point of the show. The point is to show what would happen if we all of a sudden disappeared and everything else remained the same. That way, you don't have to take into account looting, traffic clogging up interstates out of major cities, and other effects of mass hysteria. The show is just giving a hypothetical under the assumption that all else was unchanged.
Everyone knows you can't spell crap without rap.