Consider the contrasting models, and the results of their choices.
On the one hand is the Walled Garden approach of Apple, with strict (some would say onerous) screening of apps, and Apple’s ability to remove apps they deem dangerous (or subversive, or whatever it is).
On the other hand is the Android Market, which has already distributed malware more than once. One variant is Amazon’s control of Kindle Fire apps, but there’s little question that users have quite a bit of discretion with their devices.
It’s reasonable to argue that security is very much enhanced when an OS maker can wipe a malware app from every machine. But one person’s malware is another person’s mandatory feature; rootkits, for instance. How comfortable are you when that company is Microsoft? Consider it here: