If someone has announced his intention to kill you and has the means to carry out that threat then you kill him first.
Actually, my first response would be "why on earth does someone hate me enough to want me dead?"
I think the objection is not to going after people that do harm, it's about blindly doing that, without thinking or doing anything about how it ever got that far.
Terrorism is a threat. To think otherwise is naive.
You're misreading the argument. It's not that terrorism isn't a threat, it's that this threat is overblown. In the U.S., more people die from car accidents in a day than die from terrorism in a year. In africa, more people die from hunger in a day than died on 9/11 (btw, the world currently produces enough food to feed everyone, it's only politics that prevents this from happening). Terrorism in the grand scope of things is pretty small as a risk.
Now, I know the counterpoint to that "terrorism may be a small risk, but it's an unknown risk, it only takes one guy one time to kill thousands...". That may be true, but then if it's so easy for terrorists to strike, why focus so much on killing them, which doesn't do much for lowering the risk? Why not solve the underlying causes? Why not seek oil independence so military presence in the middle east is no longer required? With the money spent on iraq, america could have dramatically reduced its dependence on foreign oil.
The argument on the other side is not that these aren't causes worth fighting for, it's that the effort spent fighting them is spent the wrong way. Terrorism isn't prevented by deploying troops, it's prevented by reshaping global politics. Then another argument is entirely is why this money is misspent, and for that you only have to look at the money trails in american politics. There's no need for america to spend more on defense than all the other nations in the world combined. It's not under that big of a threat.