There is a fine line between ancestral rights and collector's rights. I appreciate that now, finally, some aspects of North American heritage are being preserved from not allowing the harvesting of petrified wood to diving wrecks and so forth.
But it is established law that you cannot implement retroactively. If I have harvested some fossils and arrow heads when it was legal to do so, then it cannot be a crime today. With that, billions in native artifacts lay in private collections across the globe, artifacts that often hold deep spiritual meaning to First Nations, some of which are being negotiated, some returned through generosity and some through law suits.
It is also established in law that chattel is clear and since the days of the Magna Carta no state can simply seize anything unless it can prove wrongdoing or right of domain such as land for military bases etc., where the practice is compensation.
The reason they are seizing this man's stuff is simple logistics. He's 91, he won't live long enough to pose a serious litigation problem as even if heirs go after them they are not entitled to damages, only the value of the collection.
I wonder how they got a warrant? It's not the FBI I question here, but a brain dead judge who would not know the US Constitution's limitation of powers provisions.