O MS define desta forma:
Windows 8 also continues to offer desktop app experiences as found in previous versions of Windows. In Windows 8, the browser that the user sets as the default for handling webpages and associated protocols may be designed to access both new experiences as well as the traditional desktop experience. This type of browser is the new experience enabled desktop browser. This guide describes how to create such a browser.
Tiles in the Start screen activate the browser in the new user experience when the browser is the default. When the browser is not the default, tiles activate the browser in the desktop. This same behavior also applies to a browser’s secondary tiles.
Não vejo um motivo (técnico) para o fato de a MS ter feito isso.