They have justice. They just aren't happy about it.
The guy whose DNA was found on their daughter, the guy whose fingerprints were found in her blood, the guy who fled the country has been put in jail. The guy who clearly lied about the crime scene. He is in jail.
There was no other solid evidence (DNA, fingerprints) that any one else was involved.
Knox sounds semi-obnoxious, like every other 20 year old college student who goes away to a foreign country and decides to get herself a man. Sex toys, condoms, PDAs. I am sure they didn't get along, but that is a far leap to murder.
For some reason the cops decided that the crime had multiple perpetrators, even though there was no evidence of it at the time.