Is pet food safe for humans?

I've heard that it's a requirement that pet food be safe for human consumption - at least here in the US. I'm not sure what the purpose of that is, unless they are concerned about young kids possibly trying to eat it.

Last summer there was a show called Around The World In 80 Plates, and one episode featured a prominent Chinese cook, Alvin Leung, who served the contestants "pet food" out of a can. It was actually human food, but humorously packed and served in a can.