Pokemon GO's staggered global release has meant that a lot of Android users have resorted to installing the game via APK file rather than wait for their local Google Play Store. Nintendo would rather you don't do that. Despite the game being free to download, and all copies connecting to the same in-game store regardless of where it came from, TorrentFreak reports that Nintendo has been quietly issuing takedown notices to Google all week, asking that search results displaying "pirated" copies be removed.
It's thumb-in-the-dyke stuff from the company, though, especially since APK Mirror — the safest, most trusted source of Android APK files — is still hosting copies of the game, on the grounds that it's a free download.
With no commercial gain to be had from stopping people playing the game, I'm guessing Nintendo is just trying to keep it in the hands of users in countries where Pokemon GO has been officially released. Maybe to cut back on stuff like the problems some Korean gamers are having right now.
Note: If you don't know what an APK is, it's "the package file format used by the Android operating system for distribution and installation of mobile apps and middleware". Basically, it's the app itself, bundled in a way that lets Android users share and install programs without the need for a centralised store like Google Play.