First because it could be potentially useful if you want to catch terrorists. For example if a suspected terrorist calls person X and person Y also calls X, X m

First because it could be potentially useful if you want to catch terrorists. For example if a suspected terrorist calls person X and person Y also calls X, X may be the terrorist leader and Y may be another terrorist. Second why would they limit it to one direction? There seems to be little if any legal or practical downside to simply collecting more data.