What exactly the difference between "private sector data" and "citizen messages"? Wikileaks published a ton of data from personal sources. Famously, the Podesta leak was from a private account and absolutely contained personal communication (about, again famously, a favored pizza joint).
I think where most people would reasonably draw the line is in the level of political power a person has over matters which impact the public interest. While I personally disagree with the leaking of Podesta's emails, the amount of political power and relevance he has makes him somewhat of a public figure. He is not in anyway comparable to your average person.
I do see a public good in providing this data to law enforcement, so that they can obtain evidence to build a case against the people who were involved. I however can not see any public good in potentially exposing random people to identity theft and criminal harassment for simply registering an account on a website.
While people could make the argument that the site has culpability for what happened so this data must be exposed, to hold that position in good faith you would then also have to say that a leak of Twitter, Facebook, and Youtube users is also justifiable. Seeing as those platforms have held worse calls for violence than any other platforms.
Parler isn't a website where all the users who register are guilty of espousing harmful rhetoric. I would agree that there are some websites that exist like Stormfront where that would definitely be the case, but ultimately Parler is simply an alternative social media site with more lax moderation than Twitter. That lax moderation unfortunately will attract a large number of bad actors who have been banned from other sites. However this still doesn't change that this site isn't anything but a social media site with a different philosophical opinion on how moderated speech on their platform should be. Which means a lot of innocent people with no political power will be harmed.
