Perhaps it is more illustrative to compare it with physical stores. The Google Play Store is one store, F-Droid is another well-known one, and there are others. Allowing only Google to install apps on your device would be like allowing people to only buy products from a store controlled by the producer of the product, reasoning that other stores might modify the product. If cheese was solely a Dutch product, the Dutch producer would get to prohibit Americans from buying cheese at stores other than a single Dutch chain, with no oversight of the American government (such as the FDA) for fear of product manipulation. They might infect it! Bringing it back to Android, I (a Dutchman) would only be allowed to get my apps from the Google store which is within some foreign jurisdiction.