The openness of the Android source code makes it possible for any manufacturer to ship a custom version of the OS along with proprietary pre-installed apps on the system partition. Most handset vendors take this opportunity to add value to their products as a market differentiator, typically through partnerships with Mobile Network Operators (MNOs), online social networks, and content providers. Google does not forbid this behavior, and it has developed its Android Compatibility Program [7] to set the requirements that the modified OS must fulfill in order to remain compatible with standard Android apps, regardless of the modifications made to the underlying OS.

https://haystack.mobi/papers/preinstalledAndroidSW_preprint.pdf