"Better the devil you know than the devil you don't." Terrorist training doesn't require a lot of infrastructure. Bombing the camps would encourage the terrorists to use other methods to conduct their training. But terrorist camps that are fixed can be watched--and that gives us our best "order of battle" information. "Order of battle" is a term of art to denote what forces are assigned to which organizations. For example, the Order of Battle of the CENTCOM theater includes, but is not limited to, the 4th Infantry Division (Mechanized), the 82nd Airborne Division, and the 101st Airborne Division (Air Assault), and so on. It's important for identifying who is behind a particular event--or who's trying to pull off an attack.