Under the FISA guidelines they can gather foreign data, but have to get a warrant for every US citizen they want to spy on. The NSA admitted they have an excess of domestic data because it's hard to filter out. So they have all this data available, and somehow we have to trust them when they say 'Oh, but we won't use it'...
To put a little perspective into things: I live in The Netherlands. A somewhat decently managed country (opinions differ!), with the highest rate of phone taps on civilians in the world. So all's not well on this side of the ocean, too.
The way I see things, there's a gliding scale between security on one end, and privacy on the other. I know the HN crowd generally gravitates towards the privacy end. But realize that a lot of people don't necessarily feel the same way. If you're a law-abiding, middle class citizen with a family, steady job and a mortgage, you're likely to give up a little privacy over security. I don't think there's anything massively wrong with that. What I do have problems with is the fact that they're being so secretive about it.