It probably is, but again, it needs to reach a certain threshold before the authorities will get involved. Threatening to reveal the identity of an internet user isn't particularly egregious, because the actual risk to that person from that information is likely minimal (is anyone going to actually hurt them?). If that person then starts to get actionable threats, then the authorities might get involved.

So the best recourse these users have is suing for libel or something if they make false claims about them in connection to the doxxing.

Unfortunately, there is no federally recognized right to privacy in public spaces, and the Internet is considered a "public space," so revealing someone's home address or identity isn't considered a violation of any law. I'm a homeowner, and you can totally find my address if you know my name, or my name if you know my address, since it's all public record. I think most people would assume an IP address is less intimate than that public record, hence why there are no laws against it.

I'm not happy about this, and I personally wish there were federal privacy protections here. I don't want my address being associated with my name as public information, though there should be a legal way to get that information when needed (i.e. a journalist doing a story on crime in an area or something). This should also apply to IP addresses, connecting an IP address to an identity should require some kind of legal measure.