Because other wise every n00b in the Galaxy would arrest everything that moved. Pretty much that simple. Read the Rules and it will give you a basic idea. Also its the A/E System. Its not complete. It will be removed Come COmbat though.
Arresting is not the same as combat. It would not make sense for every random stranger on the streets of Coruscant to be able to arrest. The government needs to grant permission to arrest in their space. Once combat is available, everyone will be able to fight, of course, but that is an entirely different feature.