Rath Rott Mony was hired by TV news station RT, formerly Russia Today, for the making of "My Mother Sold Me", a film about underage girls sold as sex workers in Cambodia where the ilicit underground trade is rife.
Rights groups have taken up the case as a rallying cry for press freedom, despite the ethical criticism of the project.
Government spokesman Phay Siphan on Thursday defended the sentence, saying Rath Rott Mony had been sentenced for "creating news to destroy the nation's reputation".
"He was punished for this fake news," Phay Siphan told AFP, adding that he had been convicted for incitement.
"This is not a punishment for speaking about sex trafficking... this is a message to the public to end fake news," Phay Siphan added.
The 30-minute documentary, broadcast in October, interviewed and followed underage girls who were sold into prostitution by their families, asking them about their lives and how they were treated in school.
The finished piece had also exposed the children's identities, though RT has now digitally blurred their faces since the story went viral.
Phil Robertson said authorities are playing a "shoot the messenger" game of a person "who told the international community about an inconvenient reality" which the government prefers to keep under wraps.
"The sex industry in Cambodia includes girls under 18 and the government officials are failing to adequately act to address it," Robertson said, adding that the conviction should be "quashed".
After a 2008 human trafficking law was enacted, Cambodia authorities have conducted brothel raids and street sweeps to stamp out the country's sex industry, especially of underage victims.
But Robertson said the reality remains much of the same, as families mired in poverty are compelled to push their children to engage in sex work to survive.
RT has not responded to AFP's request for comment.
In recent years, strongman premier Hun Sen has stepped up a crackdown on government critics and human rights activists, using the term "fake news" to dismiss opposition voices.
Cambodia is ranked 143rd out of 180 countries in the Reporters Without Borders's World Press Freedom Index.