I think they're hard-coded to appear in places that are the real-life locations of Pokemon games, and they tossed in the Googleplex for fun.
For instance, the latest Pokemon iteration is quite obviously based on France [1], so that's why Google put creatures there.
[1]: A map of the Pokemon X / Y world: http://img.gawkerassets.com/img/194c63lohgnayjpg/original.jp...