Gustave Makonene, who worked for Transparency International, was killed in July 2013 in Rubavu, near the border with the Democratic Republic of Congo.
Prosecutors said the accused killed him as he was aware of their involvement in the illicit mineral trade.
Transparency International said the sentence was too lenient.
?We followed the trial carefully and the convicts had been saying that the murder was a planned? we expected that they would get the heaviest punishment in the Rwanda penal code,? the group?s executive director in Rwanda, Apollinaire Mupiganyi, told the Reuters news agency.
On Thursday, the judge said the police corporals, Nelson Iyakaremye and Isaac Ndabarinze, did not get a life sentence as they had both confessed, pleaded guilty and co-operated with the authorities, Rwanda?s pro-government New Times paper reports.
Eastern DR Congo is rich in minerals ? many used to make computers and mobile phones ? which has fuelled unrest in the region.
Funding for the Congolese M23 rebel group was alleged by the UN to have come from minerals smuggled to Rwanda.
The Congolese army and a UN brigade defeated the group in November 2013 ? Rwanda has consistently denied backing the rebellion.