Because it is (good eye)
1st- the PS fan is wasted here (its intake is through the right side of the case instead of exhausting the case (just like the silly PSs on the bottom))
2nd-There is 3 intakes but only 1 exhaust fan
3rd- since they are trying to have flow from the bottom up (good in theory) mounting the Hdds and DVD flat like that blocks the airflow (just silly). If they mounted the Hdds and DVD on their sides air would be allowed to pass much easier.
4th 5th 6th did I mention the totally stupid way of mounting the PS?
1.) You should never use or count a PSU as an exhaust fan. Feeding it it's own supply of cold air is better for it, and will allow it to run cooler, more efficient, and last longer. Invalid point.
2.) You may be right, but positive pressure will still push a fair amount of air out, so the negative effects are much less than most think. I tested this extensively with my Stacker 830, but still, you are right that a more even approach would be better.
3.) I have to agree here, sideways would be better. But I'm also willing to bet that the negative effects are less than you would think on this as well.
4th, 5th, and 6th: See #1.
So, in short, I completely disagree with point 1, but agree mostly with 2 and 3, save the severity of the mishaps.