Because it is difficult to tell if they will act on it, but I agree. If you see that kind of anti-social, angry behavior, you should speak up, but to WHOM? Direct threats of violence should be called out as well. Police won't do anything about it until something actually happens, especially if no threat has actually been made. At the very least, an investigation of the person's social media, etc...
After Columbine, there is scrutiny over threats over email/social media that are now required to be taken seriously, but if nobody reports it, nothing happens. "Electronic" threats are actually a felony now because of Columbine.