Most Americans understand that the First Amendment's guarantee of "free speech" is not absolute. For example, you can't yell "Fire!" in a crowded theater. Another exception is obscenity. Although the Supreme Court has struggled over the years to define the constitutional boundaries between protected speech and obscenity, the standard that has been in place since 1973 was announced by the Court in Miller v. California:
The basic guidelines for the trier of fact must be: (a) whether the average person, applying contemporary community standards would find the work, taken as a whole, appeals to the prurient interest, (b) whether the work depicts or describes, in a patently offensive way, sexual conduct specifically defined by the applicable state law, and (c) whether the work, taken as a whole, lacks serious literary, artistic political or scientific value.
And even before Miller, in 1968, the Supreme Court approved greater protection for minors by upholding laws defining obscenity more stringently than would be permissible for adult content.
Congress has been mindful of those cases and the differing standards for minors in its attempts to protect minors from online porn. The first attempt, the Communications Decency Act (CDA), passed in 1996, was partially struck down by the Supreme Court in the 1997 case of Reno v. ACLU because the online pornography provisions went too far, punishing the publishing of online materials that would be permissible for adults, even though it was "obscene" by the more stringent standard for minors.
In response, Congress in 1998 passed COPA, and attempted to address the deficiencies that the Court found in the CDA by limiting the scope of the law to "commercial purposes," and to material labeled as "harmful to minors" rather than the more general label "obscene."
COPA was immediately challenged in court; an injunction was issued to prevent its enforcement. The 3rd Circuit upheld the injunction, calling the law "overbroad." The Supreme Court then found the 3rd Circuit's reason insufficient, and sent the case back down (the law remained blocked during all this). The 3rd Circuit again found the statute unconstitutional and upheld the injunction. In 2004, the Supreme Court affirmed the 3rd Circuit in Ashcroft v. ACLU, ruling that the law was likely to be unconstitutional ("likely" is the word they use at the injunction stage, before a trial has occurred). That decision observed that internet "filtering" was superior to the mechanism of COPA for protecting minors and didn't threaten materials directed to adults. Because better options existed, the Court said, Congress had not "narrowly tailored" COPA sufficiently to avoid suppressing too much speech.
Thus, the case was sent back to the original trial court with the injunction still firmly in place. That trial was then conducted, and the March 22 decision from the federal district court in Philadelphia is the outcome of that trial.
The trial judge, taking his cue from the Supreme Court, found the law to be overbroad and not narrowly tailored to address protection of minors from online porn, because filtering was superior to COPA. Judge Lowell Reed acknowledged that "protecting minors from exposure to sexually explicit material on the World Wide Web is a compelling interest." However, he found that COPA "suppresses a large amount of speech that adults have a constitutional right to receive."
The case will no doubt be appealed by the government, but given the previously unfavorable treatment COPA received during the injunction stage of this case from the 3rd Circuit and Supreme Court, it will be an uphill fight.