I mostly agree with Chris form Movie Marketing Madness on this one. The poster clearly places The Kingdom as an action film centered around Jamie Foxx, with Jennifer Garner as a secondary draw. Conversely it downplays the more political aspects of the movie and the rest of the cast. I have no idea if that is and accurate representation of the actual movie, but it seems like an effective way of selling it.
And yes, in this particular case I think the having the picture occupy only half of the poster, with the other half occupied mostly by the title and a dark background, works very well. It gives a the poster a nice distinctive look.