The attempt to rush the laws through the legislature was unacceptable, said Chloe Fussell, a campaigner at the environmental watchdog Global Witness.
"These bills are supposed to be open for public debate for months, the drafters have been promising us consultation and at the very least, the legislature should have more than a day or two to read them," she said.
The government appeared eager to pass the laws so that it could auction off new offshore blocks, Fussell said.
"But passing these laws without anyone seeing them could have resulted in an oil sector that is badly regulated," she added. "In a country marred by corruption, that would be a recipe for disaster just look at Angola and Nigeria."
Critics of the president welcomed her son's resignation, saying his removal from the government posts was overdue.
"There is a need to critically examine what he's done," said Jefferson Koijee, youth wing leader for the opposition Congress for Democratic Change. He added that Robert Sirleaf's holding of two portfolios was "causing some embarrassment" for the government.
President Sirleaf has two other sons in top government positions. Fumba Sirleaf is director of the National Security Agency, and Charles Sirleaf is deputy governor of the Central Bank, though he was temporarily suspended last year for failing to declare his assets.
Sirleaf has repeatedly defended her sons' appointments, saying they are qualified public servants.
She appeared to address the criticism obliquely in her letter to her son Robert published Tuesday.
"Perhaps it will be many years before Liberians realize the scale of the profound debt our country owes to you, but I am certain that day will come," she said. "Today, the President and the Nation thank you for your patriotic service, and wish you well."
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Corey-Boulet reported from Dakar, Senegal.