According to reports, the defendants were on a fact-finding mission investigating Thai claims of Cambodian border encroachment.
Residents say they want $1,500 per square meter, or 15 hectares slotted for themselves in the new development.
They were charged with illegally crossing the border and entering a military area.
More than 1,000 of 4,000 families have already been evicted or have moved from Boeung Kak lake area.
Richardson's visit follows a high-profile trip by US Secretary of State Hillary Clinton in November.
Members of the Thailand Patriot Network gathered in front of the Cambodian Embassy to call for the release of the group.
Meanwhile, the National Election Committee has been accused of weak enforcement of the Election law.
The reporters killed include those who supported the opposition parties, as well as the pro-government newspaper Koh Santepheap.
The charges carry a sentence of three to six months in jail and a fine of up to $500.
In a statement to the Assembly, Prime Minister Hun Sen said Cambodia had a 30-fold difference in trade imbalance.