It really doesn't matter because Prosser was in possession of stolen information. And before you say he didn't realize it was stolen, it was his responsibility to know. It is no different than if your friend loans you his bike, only to discover that the bike was stolen from someone else. Both you and your friend are guilty.
The Supreme Court disagrees with you:
In 2001, the Supreme Court ruled in Bartnicki v. Vopper that the First Amendment generally protects journalists who publish information of public concern that was acquired illegally by others. In that case, the Court held that several news media defendants who broadcast an illegally recorded phone call they lawfully obtained couldn’t be held liable under federal and Pennsylvania wiretapping laws, even though they knew the call was illegally recorded.
There are nuances, but in general a reporter is protected by the First Amendment. One exception is if the reporter participated in obtaining the material or encourged someone to obtain it:
Part three notes that courts have distinguished between a solicitation to steal information and a request for information that is already stolen. The First Amendment may not protect publishing illegally obtained material when a journalist participates in the illegal conduct, even indirectly
It might, but it's not uncommon for news reporters to be for-profit and to pay their sources for information. The issue remains on whether he solicited the misappropriation and whether he has a First Amendment defense for the publishing.
It also has to be in the public interest, but that seems to be a pretty broad category:
Reporters can also use the guide to figure out whether the illegally obtained material involves a matter of public concern, which is a key factor courts consider — though they haven’t always agreed on what qualifies. Generally, the Supreme Court has ruled that speech is a matter of public concern when it relates to “any matter of political, social, or other concern to the community.”
Source: Reporters Committee for Freedom of the Press