There are no laws in that state that cover what the officers did, and if there were there'd be a lot more people behind bars, many of whom I suspect the public would judge shouldn't be there because most tragedies resulting from omission are accidents without intent.
That's actually not true. While Maryland has a Manslaughter statute (2-207), it does not define manslaughter in the statute. It's a common law (created by cases) crime and can be voluntary or involuntary depending on intent. Generally, under the common law in Maryland, involuntary manslaughter is the
killing of another unintentionally while doing an unlawful act (not a felony), a negligent act, or
by negligently failing to perform a legal duty. Such as, say, a police regulation about securing suspects for transport in a moving vehicle. A regulation that was put in place due to the number of incidents of prisoners being injured in transit due to being not secured.
Examples:
In 2004, Jeffrey Alston was awarded $39 million after becoming paralyzed from the neck down as a result of a ride in a Baltimore police van.
In 2005, Dondi Johnson Sr., a plumber, was arrested in Baltimore for public urination. Apparently uninjured at the time of his arrest, Johnson emerged from the police van paralyzed with a broken neck, and died two weeks later from pneumonia resulting from his injuries. Johnson stated he had been thrown face-first into the interior of the van during a sharp turn. His family sued the Baltimore police and were awarded $7.4 million, which was reduced to $219,000 under a cap imposed by Maryland state law.
In 2012, Christine Abbott, a 27-year-old assistant librarian at
Johns Hopkins University, was arrested at a party she was hosting at her home in Baltimore's
Hampden neighborhood. She was handcuffed and put into a police van. Abbott later sued the officers in federal court, describing her ride as "like a roller coaster", and accusing the officers of braking short and taking "wide, fast turns" which slammed her into the walls. Police acknowledged that Abbott was not buckled in during her ride.
So that's three cases in recent history that resulted in major lawsuits against the Baltimore police, and you're gonna tell me that they couldn't have known that an unsecured suspect was at risk for serious injuries?