They create, foment, or at least allow the circumstances that cause the attacks then use those as an excuse to take away more freedom/privacy from good, decent human beings with the false promise of safety and protection. The Mayor of London, himself said we should expect more such attacks, despite the ever encroaching police and surveillance state with CCTV cameras everywhere. Way to go Mr. Khan.
I'm not sure if this is the exact quote attributed to Ben Franklin, but regardless it's true enough and we are, sadly, seeing it play out---'Those who would sacrifice liberty for security deserve neither and will lose both.'