"But the United States (like every other country in the world) needs some intelligence-gathering capabilities."

But the United States(like every other country of the world) needs control by the people who supposedly controls the country in a democracy.

If all the people is spied, and the people has no way of controlling it, then it is not a democracy anymore.

Remember that absolute power is way more dangerous than terrorists. Terrorist killed 3000 people in 9-11, Stalin, Hitler or Pol Pot killed tens of millions after taking control of the State.

Remember Hitler took control of an advanced democratic republic with the excuse of going after terrorism.