Old bottles, old cans, rusted metal, that's all garbage too, right...?
I mean somebody just carelessly left it there, so it much be garbage to be thrown away...
What about native potsherds, arrowheads, or stone tools? Somebody just left that there too, so it too must just be trash...right?
That's right, it is super-illegal to take home that arrowhead you found. Why? Because as soon as an archaeological artifact is removed from its resting place, it loses its value. Part of an archaeologist's job is to determine why something is where it is. The location of an object gives perspective to the object.
Native objects are one thing, but old cans and bottles are another. It may not even be clear that an old can or bottle is old enough to qualify under ARPA (that's the snazzy shorthand for Archaeological Protection Act) for protection.
The fourth principal of Leave No Trace is to "Leave What You Find." The intent is certainly not for you to leave obvious modern trash, but instead to leave these cultural artifacts for others to discover and enjoy in the setting that they've lived in for over a century.
--Jason D. Martin