One man's etching is another man's worthless graffiti ! One of the major problems we always had as practicing field archaeologists was defining just what is/was an "prehistoric or historic " resource (Ant. Act of 1906) and "if" something is, "is it protected by law (National Historic Act of 1964, Arch. Preservation Act of 1979.etc.,) Without getting to deep into the law(s), an object, place or thing needs to be determined to be "significant" for any host of reasons (age, condition, association with historic events,etc.,) before it is protected by any relevant fed, state or other law. Since these objects were evaluated and met one or more of those criteria-they were covered by the law.
All that said, i remember in the early 70's when that under the law, things at least 50 years old could be classified as historic, so that rusty old can dump from the '30's could overnight become and important historic resource that might need to be studied and protected! I remember my exasperated boss asking me , " can't you just find one nice example and we'll protect that one??" Several of us field arks also noted that maybe, that we and our site reports, could be now be defined as historic resources ! While most of us would agree that a axle grease signature from the 1840's and any associated wagon ruts down High Rock Canyon in Nevada, or a prehistoric mammoth kill site, or some prehistoric rock art anywhere, is protected by law---nothing will 100% protect it from someone who is determined to destroy, deface or steal it ( believe or not, someone up on the Modoc NF years ago were cutting rock art panels from the rock faces so they could sell them). No self respecting dog would take part in such activities!