Pretty simple really. It is all about control. I did a I'm Feeling Lucky search and came away with a decent enough article:
http://www.searchenginejournal.com/w...n-apple/30345/
Neither of them are perfect and it isn't as big of a gap as some would have you believe. That said, my organization bans Android devices due to the way Android marketplace works. Yes, user education solves that problem but I think we can count on 85%+ of the user community not understanding what they are doing (see any PC virus outbreak for reasoning).