The issue isn't really with Getty charging for those images. If they're in the public domain, then (as I understand it) the photographer has relinquished her copyright on the images. It's perfectly legal for other people to make money with them in that case, but that doesn't mean those other people own the copyright.
I think the issue comes into play when Getty tries to enforce a copyright they don't actually own by threatening people who use the images. That's where they've crossed a line, and I'm fairly confident that if Getty tried to sue anyone over it, the courts would decide in favor of the defendants. Of course, just defending oneself in court against someone like Getty could get expensive, so it's obviously better if the issue could be sorted out preemptively (as this photographer seems to be doing).
Getty maintains the "exclusive right" to license an image on behalf of the copyright owner (typically a photographer). The photographer does not assign ownership, just the right to license.
They use automation to find cases of license violation--using the image without paying Getty a license fee. That triggers an automated message saying "You owe us $500."
Getty doesn't (as far as I can tell) verify that the photographer owns the rights to the image, and when a person asks for that proof, Getty's response is to "Check the website" and will only submit further proof upon subpoena.
If that's true, then either the Library of Congress didn't get the memo (they very clearly specify that it's in the public domain), or this is somehow a case where "public domain" means something different from the standard definition.
It's possible they mislabeled it, but one would think they were still on the hook if they sent any DMCA notices, as they should not have any reason to believe that they were the copyright holders or in any way authorized to sue for "infringement" of the works... particularly when sending notices to the actual copyright holder.
(3) Elements of notification.—
(A) To be effective under this subsection, a notification of claimed infringement must be a written communication provided to the designated agent of a service provider that includes substantially the following:
[...]
(vi) A statement that the information in the notification is accurate, and under penalty of perjury, that the complaining party is authorized to act on behalf of the owner of an exclusive right that is allegedly infringed.
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I'm having trouble figuring out if there's any way they could have a good faith belief that they were the copyright holders here (I suspect not), though I do not know whether or not they ever sent DMCA notices containing all of the elements.
I think the issue comes into play when Getty tries to enforce a copyright they don't actually own by threatening people who use the images. That's where they've crossed a line, and I'm fairly confident that if Getty tried to sue anyone over it, the courts would decide in favor of the defendants. Of course, just defending oneself in court against someone like Getty could get expensive, so it's obviously better if the issue could be sorted out preemptively (as this photographer seems to be doing).