Tho...strictly about not being able to view pm notification while you are browsing forums on iPhone I can understand what is the reason behind this. The limitation might come from the size of the display. If the main app top bar would be available on forums aswell, browsing forums would be awfull on a 3.5" display. A change like this would probably benefit the owners of 4" display iPhones (5 and 5S) and would offer a worse experience on iPhone 3GS/4/4s. Probably at this point the owners of iPhone 5 and 5S are still a minority in KaW and that's why devs might avoid to do the change. A way would be to get rid of clan chat/world chat preview and add the main app top bar. Like this the reading forums experience would stay the same. But I don't know how many users would be happy about such change.
Basically the biggest iPhone its smaller than the average Android. You can't fit in an 3.5-4" display same options you fit in an 5" one without affecting user experience. Note that a part of screen its filled by bottom bar, bottom bar that is absent on Android being obsolete due "menu" button existence.