Logo on the back I can understand...
..it's free advertising after all.
Why the logo on the front though? Do people regularly forget what phone they have, and wonder why their iOS apps aren't on it? Do they have to sift through a pile of phones to find the right one? "I'm pretty sure the Microsoft one was ringing, I think I left it under the Nokia, but I'll have to move the Blackberry and the Erriccssonn before I can move the Apple to get to the Sony that's covering the Microsoft"