Matthew Miller allegedly ripped up his tourist visa and demanded asylum at Pyongyang's airport in April, and was sentenced to six years' hard labour by the country's Supreme Court last Sunday.
The 24-year-old nurtured "a foolish idea of spying on the prison and human rights situation while experiencing 'prison life'," the North's Korean Central News Agency (KCNA) said, in a story entitled "Detailed Report on Truth about Crime of American".
His "inveterate hostility" toward the regime was fostered by anti-Pyongyang broadcasts and publications he absorbed after arriving in South Korea's capital as a jobless university dropout, it said.
"(He) deliberately perpetrated such criminal acts for the purpose of directly going to prison..., spying on 'human rights' performance and making it known to the world".
In its report issued in February, the UN Commission on Inquiry into the North's rights record detailed a wide range of systemic abuses including murder, enslavement and torture.
The verdict against Miller came two weeks after he and the two other US detainees, Kenneth Bae and Jeffrey Fowle, pleaded for Washington's help in a televised interview with CNN in Pyongyang.
A photo of the evidence presented during his trial showed what appeared to be Miller's ripped-up visa, as well as his US passport, a tablet computer and a smartphone.
Bae, a Korean-American described by Pyongyang as a militant Christian evangelist, was sentenced last year to 15 years' hard labour on charges of seeking to topple the North's regime.
Fowle entered the North in April and was detained after reportedly leaving a copy of the Bible at a hotel. His trial has been announced but no date has been set.
Washington has vowed to "leave no stone unturned" in efforts to free the trio and has repeatedly urged Pyongyang to release them.
While being questioned by interrogators, Miller "confessed" that he tried to meet Bae in prison and together with Bae disclose North Korea's human rights conditions after leaving prison, KCNA said.
"At the trial, he admitted that he committed the crimes, prompted by his political motive to personally spy on the 'human rights' situation in the DPRK (North Korea) and disclose it in a bid to isolate and stifle the DPRK's system", it said.
KCNA said Miller's actions were undertaken "in pursuance of the US hostile policy" towards the country.
Analysts say Miller's trial is part of Pyongyang's wider efforts to capture US attention and force it to the negotiating table.
Washington has no diplomatic relations with Pyongyang and North Korea has in the past released detained Americans after visits by former presidents Bill Clinton and Jimmy Carter.
