Adam Walsh was 6 when he was abducted from a Florida department store and killed in 1981. Years later, police concluded that a convicted pedophile was the culprit. The boy's father, John Walsh, became a high-profile advocate for children and crime victims and host of TV's America’s Most Wanted.

So it's understandable that the Adam Walsh Child Protection and Safety Act would be seen as an important crime prevention tool.

The 2006 law created a national sex offender registry, increased penalties for federal crimes against children and got tougher on child pornography. But it also gave the federal government unprecedented power to keep convicted criminals imprisoned after they served their sentences.

And that's why the Supreme Court is considering whether the law unconstitutionally expands congressional power.

A basic principle of criminal law is that someone who commits an offense against society must pay a penalty. Elected legislators set parameters for punishment, based on the severity of the crime. Juries and judges then decide actual punishment warranted in individual cases. Once a fine is paid or a sentence is served, those who committed the crimes are allowed to go on with their lives, having paid their debt to society.

When prisoners are released, the government can attach reasonable conditions of probation or parole. But the government can't add punishment once a sentence is served. The Constitution — and justice — don't allow that.

But that's essentially what the Adam Walsh Act allows. When prisoners convicted of sex-related crimes were finishing their federal sentences, the attorney general designated them as "sexually dangerous" and kept them in prison, some now years beyond the length of their original sentences.

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