[Mobius]

You can't do experiments on prisoners without their consent. It's been tried, and the results have been really abusive.

Experiments? That's not as terrible as you suggest. Psychologists carrying on a research to find out why a person betrayed his faction (or worse, his species) aren't that abusive.

And they're much more productive than saying "Die, you traitor!" and putting everything to an end.

The Neo-Terran Front rebellion wouldn't have started if the Terran race wasn't militaristic and violent. If the Terrans were benevolent that rebellion wouldn't have started.

That's a major assumption. Rebellions don't start because of militarism, political reasons are the main cause.

It's much more plausibile to believe that the NTF rebellion was successful because of the fact that the Terran branch of the main GTVA authorities emerged following the collapse of the Sol jump node, and therefore their legitimate presence may have been questioned.

As I said, there is a lot of different levels in Terran society - Some civilizations are more advanced than others. If we're taking a (very dangerous) assumption that the USA are the most advanced civilization, with their fair trials and all, there are still a lot of civilizations that are less advanced, such as the PRC which still advocates some heavy-handed government stuff. In 300 years, maybe the most advanced civilization would have done away with the death penalty, but some voices will still call for it. Especially in a time of war.

In any other circumstance, mentioning the USA as a civilized country would work. Doing it when discussing death senteces, however, doesn't work because the USA aren't civilized under that point of view. And they're not going to become civilized anytime soon.

Good point about the sporadic usage of death penalties 300 years from now - that's probably the best explanation so far. :9

[DIO]

It's also possible that the current "advanced" ideas about human rights could have died out or fallen out of favor in the Freespace universe, depending on the events leading up to the formation of GTA.
For example, a event which completely destroyed the current society and system (like a nuclear war) could have happened somewhere in the 21~23rd century, leading to a formation of a more violent and "backward" society.

[General Battuta]

You can't do experiments on prisoners without their consent. It's been tried, and the results have been really abusive.

Experiments? That's not as terrible as you suggest. Psychologists carrying on a research to find out why a person betrayed his faction (or worse, his species) aren't that abusive.

And they're much more productive than saying "Die, you traitor!" and putting everything to an end.

There are ethical guidelines scientists, psychologists included, have to follow when they're recruiting individuals for experiments. You can't do an experiment or study on a population of prisoners without consent, so far as I know.

Prisoners have most commonly been used for drug tests, which is now (I believe) illegal.

A case study of an individual prisoner, as you suggested, would certainly be interesting, and those are common.