People are sentenced to prison for breaking the law (i.e. being chaotic), not for committing evil acts. Perhaps the person he killed was molesting his sister? In most D&D games, that wouldn't be classified as "evil." However, by committing the murder, he broke the law, and he's serving a sentence for it. Indeed, most rehab programs deal with enabling individuals to function within the normal society - sure sounds lawful to me.
The fact that I felt the need to follow up that statement by qualifying the next statement with the word, "Seriously...", should indicate to you that my tongue was somewhat in my cheek in the portion you quoted and that I didn't take the analogy very seriously.
In any event, I think in most societies there is a strong attempt to codify morality into the law, or at the very least to codify what is immorality into the law. I can't discuss that in detail with out getting political, but I will say that the appropriate response to the unnuanced claim of, "You can't legislate morality!", is, "What would you want to legislate instead?" Generally we don't make something illegal unless we also think it is immoral, and most certainly I think we can generally agree that murder falls into that category. We would like to think that when someone is punished for breaking the law, it is because the law justly punishes immoral behavior, and where we don't think this is so we are generally compelled to change the law so that it is so.
Our legal system takes into account that there might be mitigating factors involved, but murder is still murder - we still deem it immoral even where we might sympathize with the mitigating factors. Were we don't deem it immoral, we don't call it murder nor do we punish for it.
If you really wanted to make the argument that we don't know the alignment of the prisoners, you'd be better off arguing that not everyone that the law holds in contempt is actually guilty of the crime they are charged with. You could also effectively argue, I think, that its possible for someone to be repentive. However, whether either argument is effective in this case seems doubtful, nor does either argument help much in the argument that prisoners have an inherent right to play D&D, monopoly, tiddlywinks, basketball or World of Warcraft.