Look at it this way, the air temperature inside and outside the bumper is probably marginally different. Not enough to make real difference. It is actually a good thing that they are protected from rushing air. Think about it; have you ever tried breathing with your head out the window at 80MPH+? You would think that with all that air rushing into your face, you would be able to breathe more easily, but it actually becomes harder because of all the turbulence. The same thing would occur if you put the intakes to direct air flow. The turbulence at higher speeds would actually be detrimental to performance. If you want to do something with the bumper to get more air in there, you would be best off by adding venturis that route directly into the intakes. Notice in this picture, to the right of the drivers head.
I don't know if this is something you should worry about though...
If you really want to do something about it then do this...On the bumper cover, the fang splitters have openings which come blocked off, but can be removed. You could take tubing and route air from there, up to where the intakes are.
I for one, am using those splitter inlets to either get air to the brakes or oil cooler. I am not worried about the intakes...