Sometimes when the little guy is exposing the big guy to be a crook it is in the best interest of society to protect the source in order to force change. Many sources would not come forward if they would not be protected by the reporter. To encourage the exposure of crime sources should be protected.
There is no reason reporters should be forced to reveal their sources. The beautiful part about reporting is the fact that reporters have to seek out the truth. A reporter, most typically, finds that truth by talking to major players behind closed doors. People will stop talking to reporters if they have no guarantee of safety.