I have monitored these websites for a while and the terrorists are very aware that they are being observed so they do not give any important or specific information on the public forum plus they are well versed about hiding and their personal IP addresses, they always give each other instructions on how to do hide IP and other tricks to avoid being caught. However they use the "private mail" of these forums to get more specific and to relate important information to each other. If the forum stop to exist hence its private chat room seize to exist and hence you kill their secret communication. In addition killing a terrorist forum or a website will kill all the terrorist materials that the terrorists use and spread on the internet.
We do not have many Arabic speaking people monitoring these sites, in fact you needs at least 3 to 4 thousands of Arabic speaking people to monitor these sites 24/7 and we simply do not have these resources.
Good points J. And your opening comment was in my thoughts as I wrote.