Without their sources their credibility would become more lower than other sources and people wouldn't know which to blame. I am not sure why people think that they shouldn't reveal their sources I mean they would become more trustworthy than how they would be if they don't put down their sources.
It is important to know where the evidence that we get comes from. If we don't know who the source is then how do we know that the news is real, It could just be fake news. Fake news is when the news is not based on facts however, Is presented as accurate. Fake news would occur if we didn't know we our sources were.
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.