for all of you spinning around on this... let's get back to first principles.
1) Microsoft is in the business of making money through writing software
2) To make the most money in the software market it's a simple equation
$$$ = (number of people wanting your software)*(a $price they think is a value) - ($/hour for developer to write code)*hours*(#ofplatforms to get the max number of people)
Microsoft has always written code to this equation. It just so happened that for 20 years, the '#of platforms to get the max number of people' was a median value of 1;-). But to capture markets and steer users... they wrote to other markets if for nothing else, bait to lure people back to the big money software that MS lived on(a taste, and then reduce to a semi-useful capability to drive the best functional experience to the Win OS version camp... Think Entourage... or IE for Mac... or the old Excel for Mac.
in corporate marketplace, where demand for Office compatibility is highest (read: will pay more), with 1 main branch, and 4 sub branches of code, you can write for iOS 7 on the numbers 1 (iPad Air) and 2 (iPad Mini Retina) currently selling tablets, and the numbers 1(5s) and 3(5c) currently selling mobile phone models, as well as the most popular tablets of all time (iPad 2 and by proxy the iPad Mini), and the most popular form factor iPhone (iPhone 4 /4s) of all time.
Note: the corporate market place... where MS is focused. They have a better business reason for spending $19.99 for an app on a tablet, and they are an easier sell (sell one exec, and 2000 licenses get bought). They sell to them, they feel the consumer market will surely follow, (minimally to office365.com... which is MS's long term strategy for consumers anyway). And, iOS consumers have shown they reward good development (or just popular apps).
That's why this rumor isn't about Android, or Surface, or Win Phone, or Samsung. If you're a product manager, you write to where there is money to be made.
It's a the same reason why google writes apps for iOS... Why adobe does, why everyone does.
To sum up and with apologies to Willie Sutton:
"Why are you writing software for the iOS system?"
"because that's where the money is."