For them it is a major feature of the iPhone that you can download any app with complete impunity. No need for $150/year antivirus (for those big scary viruses windows used to get absolutely riddled with). No need to worry it's going to change your default search engine or start mining bitcoin. You know when you download something from the App Store it has been through some level of vetting. I'm not sure why so many people here are wilfully blind to this use case.
Also, the reason I specifically bring up WhatsApp is that we know Facebook will take ridiculous liberties given the chance - remember Onavo?