 Why do the webfoots only wear skirts?
All were dressed from the waist down only, in a shimmering skirt that sparkled as they moved. These are the undesirables, the incorrigibles, the nonconformists from the sixth planet. There are these web-footed people, who were hostile until they thought I was a God. The tube these webfoots use is a toy by comparison. And this is contrary to the greatest law of all—that an entity, once in existence, can never cease to exist." "As for the webfoots," the wind devil, or Sand God, said, "I will destroy them.
 The Sand God burned everything, except for the skirts.
 Skirts are traditionally worn by prisoners. The webfoots on this planet are all criminals or descendants of criminals.  The people from the sixth planet only sent skirts, when it was communicated that the Sand God had burned everything else.  The Sand God burned everything, except for the skirts.  Skirts are the traditional dress for the webfoots culture.