It is awful stupid to and no sense getting rid of the rear O2s since they play important role in PCM deciding what the injector pulse width will be for a drive cycle.
Rear O2s have zero impact to performance and should be left functional even when headers are used.

The Department of Justice and U.S. Environmental Protection Agency announced today a landmark settlement requiring Casper's Electronics, of Mundelein, IL, to pay a penalty and stop selling devices that allow cars to release excess levels of pollution into the environment, in violation of the Clean Air Act.

Today's settlement is the first of its kind, and requires Casper's to stop selling the electronic devices known as oxygen sensor simulators or "O2 Sims."
The company must also recall the devices and pay more than $74,000 in civil penalties to the United States.

These O2 sensors simulators are considered illegal "defeat devices" under the federal Clean Air Act. Casper's has sold approximately 44,000 defeat devices through retailers and from its Web site since 2001.

The EPA is currently investigating other automobile parts manufacturers and sellers that may also be making and selling similar products.
EPA has found that some companies are advertising O2 Sims and other defeat devices as a way to enhance a car's performance.

The recall required by the settlement requires Casper's to implement a mandatory repurchase program under which it must notify its customers that Casper's will buy back any O2 Sims sold by Casper's or its distributors.