Harris reportedly shared information on how to make bombs over the internet. Rights imply responsibilities, and not everyone lives up to those responsibilities. But the alternative is a society in which Big Brother gets to decide what speech is acceptable and what is not. Of course, the right of speech hardly implies that law enforcers cannot follow publicized comments and use such comments in their investigations. The threatened student shared this information with the police, which apparently prompted no action.
Parents also retain rights to monitor their children's behavior.
Children acquire their full rights only upon reaching adulthood. Harris's parents were fully capable of monitoring their son's activities, and his public documents, and taking preventative action. Apparently, they failed to do so. It could probably manage to regulate the contents of every site. There could be an official licensing agency that approved the content of each page.