And er... what exactly are they supposed to do *before* hand anyway ?
This is the free world - we aren't allowed to lock people up who haven't committed a crime. Sure conspiracy is a crime, but it's not an easy one to prove.
The truth is there is very little that free countries *can* do to prevent terrorism, which is why it's been a part of their history for the last 200 years. There is nothing new or special about current events. There has been some group or another bombing civilians in Europe or the USA every single decade since well before the US revolution.
This is just the latest in a long, long line and at no point in all that history has your risk of dying in such an attack *ever* been higher than about 1 millionth of your risk of dying because you slipped in the shower. Suicide is a much more likely way to die.
Actually in terms of ways to die... this is so far down the list that there is absolutely *no* sanity in being the least bit concerned about it. And everybody losing their minds over it is simply abundant proof that humans are absolutely terrible at risk assessment.