If you produce software just to support the hardware you sell and software that does not work on any other hardware, chances are you aren't a software company.
So as nokia is a hardware manufacturer, it produces as little code as it must to support their hardware. That means use already written code when licence permitts it, buy external OS when you need real support (mass-market), ditch internal OS when it's too bloby and heavy.
That accidentaly also means don't support hardware that you specifically don't produce (other manufacturers) anymore (n900).

So to get closer to my initial point, it may be "not moral" to use open source code to sell closed source software (and still, if the licence let you do so...), but I fail to see anything wrong in using open source code alongside closed source when all you want is providing a functionning platform.