And how does one find those targets in the first place if they have no connection with known targets? How does one find the group to infiltrate? The point is that there are many new cells that are popping up that have no connection what so ever with known terrorists. How do you find those new cells?
The idea is that limiting police powers in order to safeguard freedoms (and with them, the balance of power between the individual and the government) is acknowledged as making the job of police harder. The polices' job being harder does, in fact, mean that some number of criminals will go free some of the time, criminals who otherwise would have been caught and prosecuted. This is why absolute security is the antithesis of absolute freedom, so the question then is how to balance the two. When you safeguard liberty as your first priority and assign a lower priority to the effectiveness of law enforcement, you understand that you are taking a higher risk that you yourself will be harmed by a criminal that law enforcement could have stopped.
That's why freedom is not for cowards. The problems you worry about are well known to people who understand and value freedom. They choose freedom anyway. They also realize that the danger with which you're so concerned has been overstated. You're much more likely to be killed by a cop than a terrorist, and any factual inquiry into that based on facts would lead you to the same conclusion. Incidentally, you're also more likely to be injured by lightning. In the last 100 years, many, many more people were killed by their own governments than by any foreign enemy, so the credibility of this danger has been well established. Limited, transparent government is a time-tested manner of managing this danger.
As an aside, if terrorism is truly such a great problem and we want to reduce it in a real and effective manner, we should also stop giving excuses to the people who hate us. It's much easier for an enemy to justify their position, raise their troops' morale, and recruit new members into their brand of exteremism when they can point to concrete acts of ruthless domination the USA has actually committed. Law enforcement would certainly be more effective if its list of potential suspects could be reduced, facilitating a more focused approach on those that remain.
Anyway, the real spirit of freedom, the more value-based, individual, and courageous part that you and so many others keep failing to even recognize, let alone try to understand, is that those who understand freedom realize that a few more guilty men may go free. They consider that a small price to pay, an exchange of a finite quantity that numbers can describe in order go gain something priceless and worthwhile. It's yet another instance of failing to comprehend a viewpoint because you do not personally share it, therefore you get sidetracked by related but irrelevant issues because you have no idea how to articulate a meaningful response to it.