The atmospheric transparency does degrade toward the horizon as light travels through more air mass and leads to brighter limiting magnitude (referred to as atmospheric extinction). This clarity directly correlates to sky quality, i.e. darker skies typically have better quality toward the horizon. Your experience with sky transparency, which is strongly affected by moisture, dust and pollution content, might lead you question today's image but I strongly doubt this image has been doctored in any way to enhance the sky quality near the horizon.
With that said, composite images are sometimes created that paste in a beautiful sky on a landscape. These have identifiable traces of an unreal sky. As long as they aren't falsely claiming to be a "true" image, I treat them as art. Anyway, today's image shows what is a very nice, clear sky.
By the way, extinction is evident, just not to the level you're referring to.
Thanks for the detailed explanation
As you say, the skies in Chile are quite special, and even more so in the Atacama desert. Extinction increases greatly with the amount of water vapour present in the air and Atacama is rather famous for being the driest hot desert in the world. Regarding doctoring images - you're again right - besides contrast, white balance, some noise reduction and a little bit of sharpening, this image has not been modified in any way. I could push it quite a bit as it's a 12-frame panorama, so the noise doesn't really show up when the image is downsized so much. Nevertheless, I can provide the individual frames (and even the raw's, if it comes to that) in order to prove that this is not a "fake" image in the way described by our guest here.
I was actually expecting people to say that it's "fake" because of the sinkhole (and that's the main reason for which I marked it there). It does look like the reflection of some mountains that don't show up in the final picture, right? But if you have a look at this panorama: https://www.flickr.com/photos/27891676@N05/15252272412/
, which I took only a few minutes before the APOD image, I think it's much clearer. In this last panorama it's also quite interesting to see how the wind settles down during the shots - in the beginning the water is all fuzzy, while towards the end it looks like almost a perfect mirror. And the extinction still is incredibly low.