Under the common law, actual possession is seen as prima facie evidence of ownership — i.e., possession creates a presumption of ownership, but that presumption is rebuttable.
The U.S. Court of Appeals for the Fourth Circuit in 2006 begins a discussion of possession with:
"That possession is nine-tenths of the law is a truism hardly bearing repetition. Statements to this effect have existed almost as long as the common law itself."
Willcox v. Stroup, 467 F.3d 409, 412 (4th Cir. 2006).
It doesn’t mean whoever possesses something is automatically the owner. It means that absent evidence of superior title, possession generally suffices to show ownership.
So shouldn't the next US-American recipient of one of these notices refuse to comply on the grounds that the law is unconstitutional?