The fallacy of the comparison to heart disease/cancer/traffic accidents is that tremendous resources are spent preventing and then resolving those issues as well. They are side effects of living productively, however, so we can't rationally just ban cars. But we can put hundreds of thousands of officers on the road, and endlessly add regulations and safety features to our vehicles, etc.

Snowden is a true hero in the sense of the word -- he is in actual, physical danger for what he has done for others -- however it is not a reasonable argument when people argue that terrorism is a non-issue because <body count> less than <some other body count>.

Do you think those who wish to do you harm don't want to exponentially increase that count? If various groups could get a nuclear warhead or dirty bomb, do you really believe they wouldn't use it? If they could sabotage a major city's water supply or a nuclear power plant, do you think that would be below them?

This doesn't seek to justify the NSA's all-encompassing surveillance at all (human intelligence is where it is at), however there is an actual possibility that they have actually preventing significant events. But the problem with proactive actions is that no one gives you credit for what didn't happen.

As an aside, to add to what others mentioned about the enormous costs and economic damage that resulted from 9/11, for instance, another reason terrorism of even the smallest kind sees a dramatic response is that it is terrorism -- it terrorizes a whole populace. If someone randomly shoots people in parks, there will be a massive response because the impact is on everyone.