There isn't a morally "acceptable level" of terrorism, rape, assaults, burglaries or pretty much any form of crime. But there is still nevertheless a balance to establish between surveillance/totalitarian state and a tolerance for some level of crime. An absolute zero level of terrorism means that the society is immune to some crazy guy taking the wheel of a lorry to run over people. To get there you need an unprecedented level of controls which I'd rather not even try to picture (and which cost I'd certainly not want to pay for with my taxes).