I would check the specific state’s archaeological rights laws. They vary by state and the type of find you have. Burial sites are usually given the highest protections.
Here are some examples from Oregon.
http://www.oregon.gov/oprd/HCD/ARCH/docs/archeaology_bulletin_1_faq.pdf
This particular issue is about looting on public land. that is why these people are in trouble not because they were legally selling things found on their personal land. This is theft of all of our history.