But you've seen many PD images reshared by stock imagery companies. It raises the question of why false assertions of ownership aren't easily prosecuted, given that they constitute a kind of fraud upon the public.

That's what trademark law is. But I don't think the person who painted the portrait is going to be suing Getty for impugning his good reputation.

If something's public domain, anybody can use for anything they want, even if that's just rehosting it with your watermark.