''Nature is eventually going to take its course,'' said David Turner of the United States Forest Service. ''It is a cataclysmic event for humans. But this is a dry habitat. Fire has been sweeping through this land for centuries.'' The flames are part of a cleansing process with real benefits, a lesson the Indians learned long ago to their hunting advantage but forest officials came late to. Now, in part due to budget restrictions, they do not rush in to fight every blaze everywhere. Some isolated fires are left to burn themselves out, naturally.