You seem to have read that post and come to a conclusion completely the opposite of mine. I read it as people (other than police) are not sane and reasonable, and the notion that it is an impossible-to-solve nihilist position is a problem amongst those who are not sane and reasonable.
The statement can be read by inversion: Successful terrorist attacks are not the fault of police or intelligence services. Police should not believe they should catch absolutely every incident before it happens, and those spreading such a belief are prone to over-reaction when the police DO miss something.
I think part of the problem is the segue into the next idea: In the end, its such a needle in a haystack sort of problem that its entirely unreasonable to think they can ever be prevented, therefore any acceptance of that reasoning that starts with they should be able to catch it, inevitably leads to excessive measures, and guarantees more excessive measures later WHEN the next one happens.
Move it around and then re-read: When people believe the police should be able to catch all of them, it leads to excessive measures since catching all of them is, in reality, not possible. When the next event happens, it leads to more extreme measures since the police obviously should be able to catch everything. "Acceptance of that reason" doesn't refer to the statement preceding it, but to the statement following it (despite the utterly confused way it's phrased).
At least that's my take on it. Maybe the author meant it the way you read it, but that determination would really be up to the author to clarify at the end of the day.