Hmmmm..
Because using your operating system´s theme as app skin is something that is taught in every single UX-desinging class. Apps that seemingly go with main operating systems theme are regarded as something that every single devel should be encouraged to do :|

For more about this follow recent market speeches about windows mobile and how user can´t see where apps start and where OS ends.

I do not totally agree or like this but to me it´s something that seems to be something that most UI designers want.