App Store vs Android Store
Of course it's in Google's interest to let users sideload all the apps they want and then blather on about "openness": They don't have to support the hardware and don't have to field complaints from parents.
Apple gets flack for malicious apps (witness the retards who jailbroke their phones and left the default root password with ssh wide open, etc.) and has to pay the cost (opportunity costs or quality perception costs, Apple Retail store visits, customer support calls, etc.) for supporting the hardware so it's in their interest to do something about the software if they can.