A customer at work yesterday complained that David Cameron was "giving terrorists ideas" by talking about possible things they could do. I've heard similar sentiments before. I tried explaiming that poisoning water supplies, breaking internet and phone lines etc was not IS's MO.
IS and their fellow bands of deeply unmerry men (and some women), like gun fire, explosions and other flashy modes of mayhem. Sure, cutting internet lines will cause disruption, but it's not showy and it's not scary to anyone but a few millenials.
Cameron knows this. Cameron knows IS et al are unlikely to listen to him and go "wow, what a good idea, let's try that instead of blowing ourselves up!"