That's why the film contains so many political speeches in its background soundtrack - they're meant to be there to draw the link between what is unfolding on the screen and what is happening in the real America today.
This film is rich with different layers of irony; the most obvious of which is the fact that the men being killed are being punished for simply doing what their killers do themselves day in and day out - commit crime and steal from others.
It seems to me that the mafia bosses are symbolic of the politicians who blame the business sector, and then seek to punish them, for what are actually failings of the system that they continue to prop up and exploit for their own ends. And just consider the fact that after killing several men for being thieves, these exact same mafia bosses then try and rob Brad Pitt's character of what he is actually financially entitled to from them.
The reason both Obama and Bush are heard at different times in the film is because we are meant to realize that this problem is not exclusive to either the left or the right, it is about what America, as a whole, has allowed itself to become as a nation. And also to highlight the fact that both left and right have allowed this problem to persist and grow.
Brad Pitt's speech at the end of the film is really the essence of what this film is about - a cynical examination of the death of the American dream and American idealism.
I think that in time this film will come to be more highly regarded as a clever piece of commentary on present day America - and when it is viewed in that light (rather than as a gangster film) it makes much more sense to the viewer.