Fees can be extracted in many ways:

  While Android is open source, the Google
  applications, like the Play Store, Gmail,
  Google Maps, Google Play Services, and others
  must be licensed. This licensing agreement is
  called the "Mobile Application Distribution
  Agreement" (MADA) and comes with tons of
  restrictions.[1]

This is only one example and the monetary considerations are unknown.

1 - http://arstechnica.com/gadgets/2014/02/new-android-oem-licen...