1) A test to see how the journalist will react and to flush out more information. I.e. force him to disclose more information so they know what he has.
2) Another test is to test public opinion. So far we have seen nothing but defensiveness and accusations from the White House. That bill to de-fund the NSA had some legs but didn't get off the ground. If there is no reaction that is a good benchmark, they don't have to stop and do anything. They could even push the envelope further and probably not get an real repercussions.
3) A warning is a powerful thing. It is message to all other journalists working with NSA sources. "Are you prepared to open your family to harassment, black lists and 9 hour detentions during travel?". As some mentioned in the comments, even the Mafia has basic engagement rules to leave family members alone. That tells one the level of engagement and the mentality of government actors we are dealing with.