I've seen that same picture in a book. Not sure if I could locate it, but the caption was that they were war weary and damaged airframes that were beyond repair. As such, they were bulldozed onto a pile after critical spare parts had been removed. Space was a bit limited on Tinian, Saipan and Guam, so I doubt they had room for hulks, and I would imagine that the cost and logisitcs of returning them to the U.S. far outweighed their value as scrap.