These are called style guidelines and iOS has them too [1]. If you don't adhere to some of them, you won't even get your app in the App Store. So you need to design your app in an "iOS" way, just like here Windows Phone devs should design in a Windows Phone way.

http://developer.apple.com/library/ios/#documentation/userex...

it's much easier to design an app if you can say "don't bevel anything that isn't a button" rather than "don't put anything in a rectangle that isn't a button".

That's a strawman. No one said: "don't put anything in a rectangle that isn't a button".

if all of the buttons are coloured rectangles, how do you differentiate a non button rectangle?

That depends on what you want to show, of course. What kind of example do you have in mind? What kind of information are you showing?

Here are some examples of non-button rectangles:

http://www.windowsphone.com/en-us/store/app/weather-clock/ec...

http://www.windowsphone.com/en-us/store/app/weather/171f3a75...

The differetiators here seems to be that 1) the rectangle is not a square 2) the rectangle has rounded corners

edit: and I guess 3) the rectangle is not "flat", it has a gradient, 4) it's not colored, it's transparent grey, unlike any clickable rectangle in the system. .... (basically, it's fairly easy to differentiate between a clickable square tile and a pure informational rectangle)

Those UIs look quite cluttered and unclear to me, but I suppose a weather app isn't the best benchmark.