I haven't seen the programme, but have read bits about this story on line. There appear to be racial and class dimensions to the case (i.e. the man she killed was white, an estate agent, church volunteer and had some status in the community, she was mixed heritage, poor and considered to be of the "underclass.")
Having grown up in America, it's my view that there tends to be more of a "black or white" (no pun intended actually!) view of crime and punishment, which is reflected in jury decisions and in public responses to crime. There also tends to be less sympathy or understanding for factors that may have impacted on either perpetrators or victims of crime (e.g. previous abuse, neglect, mental ill-health, etc.) particularly if the person isn't "seen" to be a "good person," (e.g. not the right, class, ethnicity, morality, religion, social status, etc.)
In this case, I've read alot of comments where people have no truck with anything that may have happened in Cyntoia's past that could have contributed to her actions or the issue of being sentenced as an adult for a crime committed as a minor. They see it only as one person killing another in "cold blood" so they should receive the maximum penalty for the crime. They simply aren't interested in any "complicating factors," imho, because that wouldn't square with their very simplistic world view that everything is absolute, right or wrong with now in between.
Generally, that simplistic view tends to be sexist, racist, homophobic, classist, disablist, etc. as well of course.