People should sign because if we stop showing terrorist killings on the news and online, it stops making the perpetrators martyrs for the cause and encourages people to do the same. Also it makes people feel scared and stops people from living their lives because they are scared that they are going to be attacked in public.
It is important because it alienates the victim's families and makes people feel less safe in their communities which could lead to extreme measures.
By not making them public and just telling the local community by whichever means makes people feel safer and makes the community aware that they could be in some danger without making everyone else scared.