Thanks for pointing that out, it's a reasonable concern. That is indeed the US Attorney's argument.
Just to clarify, it is not necessary to be on a recognized archeological site to be illegal, intending to collect artifacts on federal land is a crime whether a recognized archeological site or not.
But in this case they were in camping in "Salmon River Canyon, near the mouth of Graves Creek, which is in Idaho County" on land "administered by the Bureau of Land Management".
Photos of Graves Creek: http://www.nww.usace.army.mil/dpn/f616a.htm
It's not a recognized archaeological site beyond the interpretation that every location in the US is an archaeological site.
To be sure, there are arrowheads and/or evidence of human habitation on nearly every square mile of this continent. Just having the possibility of arrowheads doesn't make something a real archaeological site unless we bend the definition so far as to not have any meaning.
-- UPDATE --
To save time, I'll now provide a rebuttal to my own argument. Here's the BLM site on archaeological research in the area: http://www.blm.gov/id/st/en/fo/cottonwood/lower_salmon_river...
They cited that page to justify the claim it was an archaeological site in the article about the prosecution: http://www.blm.gov/id/st/en/info/newsroom/2009/june/archaeol...
Eh, OK, I agree with you!