The US-based rights group said the charges against Leang Sokchouen had changed during appeal to incitement under the new penal code.
Leang Sokchouen was given a two-year prison sentence and $500 fine in August 2010.
Union leaders say the draft of the law will tighten restrictions on organized labor and create harsh punishments for unionists.
Experts say as little as 30 percent of the country’s forest cover remains, while logging continues to be a problem.
The ceremony marked a defeat for residents who have helplessly watched Shukaku pump fill from the bottom of the Tonle Sap into the lake.
The International Center for Prison Studies rates Cambodia’s prisons as one of top 25 most overcrowded.
In 2010, the government interceded in 160 cases of human trafficking, smuggling or labor exploitation, according to official figures.
The law is to move from the draft stage at the Ministry of Interior for approval by the Council of Ministers in the near future.
Cambodia is considered “not free” by the press freedom watchdog Freedom House.
Human Rights Watch, Amnesty International and other major international groups have called on the government to spike the law entirely.
The main purpose of the visit (his fifth) was to assess how well parliament functions in upholding the rights of ordinary Cambodians.
The rights group Adhoc has so far recorded at least 124 arrests in land disputes this year, with at least 18 people now in detention.