Or perhaps Nintendo is doing as they said and expanding their characters to new partners and decided to let people have a laugh and some fun. They have also talked about taking advantage of the mobile market (not by releasing games but to expand upon the popularity of handhelds) and what better way to do than integrate Pokemon into a popular app everybody has on their phone.

Source for what they said:
http://www.ign.com/articles...

First, you have to be zoomed in enough to see the pokemon. Most show up when the zoom distance is set to 1000ft. But yes, most of the world is pokemon-less

Second, the biggest concentration areas are the cities which were used to base the game maps off of.