Great question - you get the gold star today! And Shirley Corriher, author and food scientist, provides the answer in Cookwise (Canada, UK). She says water flows in and out of cells, but not in a random manner - generally it flows towards the most concentrated solution in the tissue. So when you rub salt on the surface of meat, the salt and juices at the surface combine to make a very concentrated solution, which acts as a magnet to the other liquid in the meat. If the meat is in the sun, the water evaporates as it gets to the surface instead of diluting the salt, so the salt continues to draw moisture from within the meat.
But, the free liquid in the cells of meat is actually very concentrated with dissolved substances, Corriher says, more so than any brining solution you would make. So the less concentrated brining liquid is drawn into the meat towards the more concentrated liquid within the cells, increasing its juiciness.