It actually isn’t, at least how it has been determined in case law. Dan Ellsberg, famed leaker of the Pentagon Papers, testified about this just a few days ago in the Assange extradition hearing.
Whistleblowing isn’t stealing or the illegal revelation of secrets. You have to actually prove it’s not whistleblowing for it to be possible to consider it theft.
That’s why the US government rushed to add last minute new indictments against Assange for violating the Espionage Act by putting US interests in harm’s way - they knew their case in terms of possession, theft or publishing was complete b.s.
In the legal sense of "obstruction" (which is narrower than the colloquial sense) it's different. The law makes a distinction between refusing to help law enforcement, and actively hindering or deceiving law enforcement. It's not a crime to refuse to provide helpful information to law enforcement. The government can serve you with a subpoena for that information, but that's a civil process. While you can be held in civil contempt for refusing to comply with a subpoena, it's not a crime, and there are lots of circumstances under which people are "privileged" from having to comply with such subpoenas. (Lawyer-client, doctor-patient, between spouses, etc.) Journalists are generally held to be protected by a similar privilege.
Those privileges don't protect you from actively assisting in a cover-up or concealment of evidence of a crime, which is itself a crime. A lawyer doesn't have to tell a prosecutor where to find relevant evidence, but he can't help his client hide that evidence in the first place.
Whistleblowing isn’t stealing or the illegal revelation of secrets. You have to actually prove it’s not whistleblowing for it to be possible to consider it theft.
That’s why the US government rushed to add last minute new indictments against Assange for violating the Espionage Act by putting US interests in harm’s way - they knew their case in terms of possession, theft or publishing was complete b.s.