I'm guessing that he means a reporter's sources are protected under the freedom of the press granted by the first amendment. For a reporter, having his address book exposed could be quite a bit more damaging than for the average user.
Exactly. It's an issue because, if someone asks him for his sources, he can say no: if someone asks Path for his sources (via subpoena), they might not be able to say no. So this upload means he can no longer guarantee sources their name will never come out.