Apple is notorious for keeping pornography off.
Google? Not so much. Why?
How many applications do you have on your mobile device? If one suddenly lodged itself on your Android, icon and all, would you notice? You have some difficulty (brought to you by Pfizer, Carl's Jr., whomever) with your phone, your life, your car, etc. You download an app which promises a solution. No solution. You forget the app. The app runs a protocol. Lodges an application programming interface which enables the app creator to insert behavior in your activity. How do you know? What do you know?