It's a very good question and should be the focus of criticizing the NSA's programs unless we have evidence that what they did was malicious (blackmail, etc).

But I don't think they mean that when they say trust us. They typically mean: trust us, if you knew the threats we face and the fact that this is effective, you'd think the risks were worth it too. In addition they mean trust us not to abuse that power, but that is the smaller point.

In fact, I suspect they'd argue the risk for abuse is outweighed by the consequences of terrorism(a statement I'd disagree with), even if you aren't sure they are honest.