Why is is that some people have such a hard-on for other people's misery? It seems that every news story about a minor infraction is met with snivelling cretins vomiting sadistic fantasies of prison life for the subject of the story.

I think you've answered your own question. It's just this kind of story makes it socially acceptable to express it (and in this particular case the subject's marked lack of empathy seems to excuse us taking the same view).