Journalists, in spite of sometimes questionable practices, do serve an important function in many levels of society. Journalists cannot be assured of getting full stories and dissenting opinions if their sources feel that they can be easily compromised. Sources deserve protection because they might be legitimately trying to help (whistle blowing), to the chagrin of the state.
No, journalists are not justified in protecting sources under subpoena. I believe that it is unjustifiable that journalists protect a source that could potentially be able to get a murderer or rapist off the street. These sources could also have inside knowledge that could help bring down crime rings and have already taken a step by talking to someone.