Boston civil-liberties lawyer Harvey Silverglate calls his new book “Three Felonies a Day,” referring to the number of crimes he estimates the average American now unwittingly commits because of vague laws. New technology adds its own complexity, making innocent activity potentially criminal.

Mr. Silverglate describes several cases in which prosecutors didn’t understand or didn’t want to understand technology. This problem is compounded by a trend that has accelerated since the 1980s for prosecutors to abandon the principle that there can’t be a crime without criminal intent.

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Other misunderstandings of the Web criminalize the exercise of First Amendment rights. A Saudi student in Idaho was charged in 2003 with offering “material support” to terrorists. He had operated Web sites for a Muslim charity that focused on normal religious training, but was prosecuted on the theory that if a user followed enough links off his site, he would find violent, anti-American comments on other sites. The Internet is a series of links, so if there’s liability for anything in an online chain, it would be hard to avoid prosecution.

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These miscarriages are avoidable. Under the English common law we inherited, a crime requires intent. This protection is disappearing in the U.S. As Mr. Silverglate writes, “Since the New Deal era, Congress has delegated to various administrative agencies the task of writing the regulations,” even as “Congress has demonstrated a growing dysfunction in crafting legislation that can in fact be understood.” Prosecutors identify defendants to go after instead of finding a law that was broken and figuring out who did it. Expect more such prosecutions as Washington adds regulations.