According to Morton, ICE's seizure of eight domain names prompted 80 other sites selling counterfeit goods to shut down on their own.
"I've never seen that kind of deterrence come from a single law enforcement action before," Morton said at the State of the Net conference in Washington on Tuesday.
The firms also wrote in support of the Combating Online Infringement and Counterfeits Act (COICA), which stalled in the Senate during last Congress after passing the Judiciary Committee and will likely be introduced again this session by Sens. Patrick Leahy (D-Vt.) and Orrin Hatch (R-Utah).
The bill would make it easier for the Justice Department to shut down websites providing pirated materials by shutting down the domain after receiving a preliminary court order. The bill has broad support from the U.S. Chamber of Commerce as well as the clothing, entertainment and software industries.
However, critics including the Electronic Frontier Foundation, have argued the bill amounts to online censorship and would infringe on Americans' free speech rights.
Sen. Ron Wyden (D-Ore.) has been COICA's most vocal opponent on the Hill, calling its approach too heavy-handed and vowing to put a formal hold on the bill to prevent a floor vote. Two of his home state's largest firms, Nike and Adidas, are both supporters of the legislation.