As you wrote yourself, terrorism is a minor issue in the US. Most of the fear is fueled (and some of it created) by the government, because it's so convenient for manipulating the population. Therefore the government itself cannot be driven by the fear of being accused of going soft in this matter, because if that was the case, it'd simply stop perpetuing this fear of terrorism.
I'm sorry, but there is no cheap way out for this goverment. It's fundamentally evil and corrupt, it uses terrorism to keep a certain elite in power and to channel taxpayers' money to the pockets of the military-industrial complex. That's the only logical explanation I've seen so far.
The surveillance state was never intended to stop terrorism, it is the true terrorism. This malice cannot be explained by incompetence.
I don't like it, and I don't agree with it, but I acknowledge that if a 2nd 9/11 level event were to occur after the government relented and imposed reasonable limitations on data collection that the party out of power at the time could easily scream bloody murder and take all 3 branches of government in a single election cycle. I'd previously only expected this from the party of Kang, but Obama's stance on drones and NSA surveillance has revealed the party of Kodos is no better.
Which is to say I think the surveillance state is a symptom more than the disease. America has lost all sense of perspective.
~30,000 automotive deaths (of which ~10,000 resulted from drunk driving) annually.
~11,000 gun deaths.
~6,000 deaths from falling off ladders.
~3,000 people died on 9/11.
Ergo we should ban ladders, guns, alcohol, and cars: it's the only way to be sure, no?