It's got to be a joke on the innumeracy of [well, you fill in the blank.]
There are hundreds of billions of galaxies. Each has hundreds of billions of stars. Multiply those two numbers together and then multiply that by all the thousands of rocks circling each star...
But aliens are watching what we do to this one planet???
You'd think they'd be happy for us to kill ourselves off so they can take it over and fix it with their superior ecological knowledge.
Update: The paper that started it all. PDF is interesting reading. SF gave us all this years ago.