It implies they are terrorising for a reason. They have something to achieve or an ideal to serve and inflicting terror is the medium. Random school shootings are acts of terrorism if they have a motive or are carrying out such acts in the name of an ideal (most common being religion). We have to draw a line somewhere and sometimes people are on both sides of that. But we still have to label these acts as terrorism or as something else to prevent a slippery slope where every act of violence becomes terrorism to the point where a gas station robbery is an act of terror. Somewhere along the lines we have to decide what is terrorism and what is normal crime.