The US justice system is a joke
Years ago before most people alive today remember (I barely remember) there was a "bleeding heart liberal" era of criminal justice in the US where there was more emphasis on rehabilitation than incarceration. Then the pendulum swung the other way--and just kept going (even though crime has been falling for 20 years now). Now it's all about "throwing the book at 'em", "getting tough on crime", "sending a message" and "zero tolerance".
The problem is--and this isn't a "bleeding heart liberal" thing at all--you simply CANNOT "throw the book" at EVERYONE. The prison space is limited. The money to incarcerate people is limited. It is not that these girls didn't do something serious, but if you insist on incarcerating them for years in adult jails for a crime they committed when they were 12 and mentally ill, you have to let someone else OUT, who--chances are--is far more DANGEROUS.
It's always a bad thing when a case gets a lot of attention from the media and has "sexy" or "evil" element to it like Slenderman. There is a much less well-known case I saw recently where two girls barely older than this were kidnapped, beaten, raped, and forced to work as prostitutes by a vicious, scumbag pimp. The pimp got an 8-year slap-on-the-wrist and is probably walking the streets right now. There are people that have committed murder that did no time. There are dangerous sex offenders in every US neighborhood. Yet, there's a good chance they're going to "throw the book" here and "send a message" (apparently the message is you shouldn't have schizophrenic delusions of internet memes). It's really disgusting and shows what happens when an entire country starts thinking with their spleens instead of their brains.
"Let be be finale of seem/ The only emperor is the Emperor of Ice Cream"