If you've ever submitted an app to Google playstore, you'll notice that nobody from Google bothers to test it, whether it is bugged, loaded with worms, malware or if its some scam app.
The app is only reviewed if there's complaints and stuff, but whats the use now? the infection is already spread.
Compare this with Apple and Windows (Closed source, they do not allow rooting and stuff, APple does to a certain extent but not as much as android), each app is manually reviewed before its made public. You try to upload some worm and they'll catch you for real.
The difference?
I can spread a very nice game, loaded with a nasty worm that hacks the users data and stuff under android.
Try the same thing with Apple and wp8 and I won't be able to breach the gates.
Besides, Playstore has a 1 time fee of $25 to publish unlimited apps, all are made live without any checking and stuff. Meaning, pay $25 and infect countless souls.
Apple needs a yearly payment, wp may have changed policies, but from what I remember they needed an yearly payment too, they did drop it to a 1 time payment of $19 but I don;t know if its still valid.