Under Section 6, subsection (b), the act states, "No person may sell, purchase, exchange, transport, receive, or offer to sell, purchase, or exchange any archaeological resource if such resource was excavated or removed from public lands or Indian lands" in violation of the act's general restrictions and permitting requirements.
However, with respect to the heavy fines and penalties that can be assessed for violating the law, the act states that "nothing in subsection (d) of this section shall be deemed applicable to any person with respect to the removal of arrowheads located on the surface of the ground."
This language is sometimes called the "Carter clause." President Jimmy Carter, an arrowhead collector himself, was in office when the Archaeological Resources Protection Act passed.
John Fryar, a now-retired Bureau of Indian Affairs criminal investigator, told an Arizona newspaper in March 2005 that's why surface finds were exempt from the stiff penalties.
"We didn't want to make the sitting president of United States a convicted felon," he said. "Also, there was a Boy Scout merit badge for collecting arrowheads, and nobody wants to make Boy Scouts criminals."
I'd never heard of this myself. It mentions further down that other laws are supposedly in place to protect surface finds.