I'm not arguing with your logic. I no longer recognise the place I was born and spent my formative years.
My point is where do you draw the line and how do you draw the line.
London has been suffering from terrorism since the early 70's, a lot you do not hear about, much the same as you do not hear about many quiet FBI run ops in the US. The Israelis have been dealing with it since the birth of Israel, and as you rightly point out they have problems stopping it - I worked with some of their people for a couple of years sharing expertise. As long as you want to maintain a free society you will never fully prevent actions like this happening - what you do is make it so painfully obvious to any potential perpetrators that you will turn their life into a living hell and grind up their bones when you catch them that they will think twice about conducting such acts.