Well sentencing vs. punishment.
Was he to be incarcerated for X number of years and then executed, or was he being held while they were building a gallows so that he didn't run away? It's question of red tape.
You wouldn't need death row if you could trust criminals to show up for their own executions when they were arranged to happen... HEY!? That's an episode of TOS! ..AND TNG!
But holding prisoners till execution is just gravy because just because you're not ready to cut their head off with something sharp does it really mean they have to languish in a cell? Well actually it does, which is why they invented the system of Law they di in the 2000 AD Comics.
In that mind, Suder wasn't planning on being held for X number of years either, which cannot be at all legal. he was given life via the captains discretion and not a day in court.