While that may be, it may also be due to sampling. Reading the page that hosts this image, it indicates that the image was created from data acquired over 500 orbits, and over the years 2000-2004 (it's a composite image). So presumably they merged areas from images that were essentially cloud-free. It may be that the least cloudy periods in California are when the Central Valley appears dry, such as in winter. However, it's also worth noting that California has been fighting a drought over most of that period, so the observation that the Central Valley looks dry may still be applicable. (Also note that they apparently didn't use images from autumn; the forests on the East Coast turn an observable orange-reddish brown in autumn.)
Right you are. I am the person who created the image and, not surprisingly, the clear days fall generally in the dry seasons of the different regions. I tried to find the best scenes of California when there were some crops showing (always something growing somewhere there). Right again about the East Coast. The autumn is generally clear, but the Appalachians are orange-brown. My goal was to get a clear summer across the country - not so easy.
For anyone interested, the really high resolution version of this is at 278m and is a 1.9GB TIFF image (360MB compressed) - 34500x18000 or so. The high quality jpeg is about 55MB.