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).