But the commission usually requires companies to enter into a legally binding order when there is concern that the law is being broken. The commission required Google to accept a binding order on Thursday in a separate investigation involving the company's use of industry-standard patents.
Rosch said the acceptance of a voluntary commitment in the case over Google's search business is an "unjustified and dangerous weakening of the Commission’s law enforcement authority."
He argued that without a consent decree, Google could revive the practices at any time without penalty.
"In other words, after promising an elephant more than a year ago, the Commission instead has brought forth a couple of mice," he wrote.
Rosch predicted that in the future, other companies under investigation will demand similar terms, and that if the FTC pushes for a formal order, it will seem like Google got preferential treatment.
Sen. Patrick LeahyPatrick LeahySenate confirms first nominees of Trump era Senate gears up for battle over Trump's CIA pick Overnight Tech: Meet the key players for Trump on tech | Patent chief staying on | Kerry aide goes to Snapchat | Uber's M settlement MORE (D-Vt.), chairman of the Senate Judiciary Committee, said in a statement that he is "disappointed" that the FTC "relied on simple, voluntary commitments from Google to end certain practices that a majority of Commissioners found to have raised strong concerns about impeding innovation."
At a press conference announcing the decision, FTC Chairman Jon Leibowitz described Google's commitments as legally binding.
David Balto, a former policy director of the FTC's Bureau of Competition who has received funding from Google in the past, argued that if Google breaks its voluntary commitments, the FTC could sue the company for engaging in a deceptive business practice.
"They secured about 95 percent of what they would have in a formal order," he said.
He argued the agency was "hamstrung" because a formal order would have required the commission to conclude that Google had broken the law — a difficult case to make, he said.