Scenic nationalism and the economic "worthlessness" of an area still largely determined the eligibility of a piece of land for national park status.  If the land seemed adaptable to more profitable uses, preservationists had difficulty protecting it.  The Lacey Act of 1906 helped the preservationists but at the same time compounded their problems.  The Lacey Act created a kind of halfway status for some public lands by allowing the president, without congressional approval, to set aside "historic landmarks, historic and prehistoric structures and other objects of historic or scientific interest" as national monuments.  Teddy Roosevelt had aroused a good deal of controversy when he used the act to reserve 800,000 acres of Arizona Territory as Grand Canyon National Monument in the 1908.  National monuments were closed to private land entries, but unlike national parks, many of them remained under the Forest Service.  And the Forest Service administered them as multiple-use areas -- that is, it allowed grazing and lumbering on them.
 

Read more about it in White's "It's Your Misfortune and None of My Own": A New History of the American West,Chapter 15, "Centers of Power."