Earlier quoted context omitted.
If a criminal releases 100,000 credit card numbers, we don't all of the sudden consider credit card numbers to be public. We try to limit the spread of the leak, if possible, and shore up whatever security lapse occurred. Nowhere in that scenario to we begin to consider credit card numbers public. So it is with birthdays. There isn't any government organization who will distribute that information.
You most certainly do consider stolen credit cards public! You don't just limit the information and hope for the best—you attempt to inform everyone affected of the breach and try to get everyone to change their card number (because it's public at this point).
If all credit card numbers were stolen and distributed, we would still have considered them private before that.
The same is true for these birthdays. They were private, then stolen and made public. That they are now public doesn't retroactively make the theft okay. Theft is theft.
The lesson here is to tighten security, increase security education and awareness, and increase investigation into the most egregious of these crimes so that violators can be brought to justice.