It was actually the government that argued that the existence of blocking software would actually make the CDA constitutional -- because it would give content providers a means to comply with the law. Both the Philadelphia federal court and the Supreme Court rejected the government's argument in favor of blocking software. Judge Dolores K. Sloviter, writing her ruling against the CDA in June 1996, said:
The government makes what I view as an extraordinary argument in its brief. It argues that blocking technology needed for effective parental control is not yet widespread but that it "will imminently be in place." Government's Post-hearing Memorandum at 66. It then states that if we uphold the CDA, it "will likely unleash the 'creative genius' of the Internet community to find a myriad of possible solutions." I can imagine few arguments less likely to persuade a court to uphold a criminal statute than one that depends on future technology to cabin the reach of the statute within constitutional bounds. [full text of the decision -- 248 K file]One year later, Justice Sandra Day O'Connor wrote in a dissenting opinion from the Supreme Court, describing how she believed the CDA could be constitutional in some respects because of the existence of blocking software:
I write separately to explain why I view the Communications Decency Act of 1996 (CDA) as little more than an attempt by Congress to create "adult zones" on the Internet. Our precedent indicates that the creation of such zones can be constitutionally sound. [...] This user-based zoning is accomplished through the use of screening software (such as Cyber Patrol or SurfWatch) or browsers with screening capabilities... [text of the decision]This is what the government had argued: that the CDA was a "zoning" law which could be enforced through the use of software such as Cyber Patrol. (Justice O'Connor ruled, however, that the CDA was unconstitutional for other reasons.)