Retroactive punishment for $5 worth of drugs: two years of torture and deportation

NPR’s All Things Considered has a disturbing and powerful two-part investigative report that is airing today and tomorrow: Immigrant Detainees Tell of Attack Dogs and Abuse

Zwerdling’s first report looks at the case of Hemnauth Mohabir, a native of Guyana. In the spring of 2002, Mohabir returned to Guyana to visit his mother, who was ill. On his way back to New York that April, an immigration agent at Kennedy International Airport noticed Mohabir had a criminal record: Six years earlier, he’d been convicted of possessing about $5 worth of drugs. The judge fined him $250 for a misdemeanor and let him go.

Because of that past conviction, Mohabir was deported to Guyana and banned from ever coming back to the United States. But before returning to his native country, Mohabir was detained for almost two years at New Jersey’s Passaic County Jail, where he alleges that guards taunted and beat detainees and terrorized them with dogs.

The documented evidence seems pretty clear in the report that there are significant abuses going on there.
This is sick. A past conviction for a small drug possession misdemeanor does not give our government the right to imprison someone for years and subject them to intimidation and abuse. This is not about homeland security – the $5 worth of drugs didn’t make him a security threat. It’s not about punishing criminals – a judge had already given him the legal penalty – a fine. It’s not about evicting illegal immigrants – Mohabir was in the country legally and had a green card. And we, the taxpayers, had to pay lots of money to detain him for two years.
Can anyone give me a valid or logical reason in a free country for this kind of activity?