They shouldn't be able to threaten legal action for those who acquire public domain images elsewhere, just because they choose to fraudulently license them.
Agreed. It was a (shitty) mistake and they apologized immediately. This was not part of the court case, and Getty has never argued they should have been able to do this.
I imagine they automate copyright takedown notices, and there was a bug somewhere. It's not great, but I don't believe it was malice.