They should be turning in the terrorists all around the world.
Why would they do that?
Most Muslims in the countries where the terrorists like to hang out are familiar with brutal governments that imprison people without trial, torture them, and summarily execute them. They've heard that the governments of the West are different, but they're sceptical. And when the United States government does get its hand on alleged terrorists, what does it do? It imprisons them without trial, tortures them, and summarily executes them. (The big difference seems to be that the U.S. will only kill you if they haven't captured you; after capture, you're fairly safe.) Meanwhile, they've been hearing rumours that the terrorist attacks are really being done by Mossad, not by Muslims at all. And they're going to turn in their friends and neighbours just because they're getting involved with radical groups?
You and I know that Al Qaeda is an organisation of murderous terrorists, that 9/11 was not a Jewish plot, and that Guantánamo Bay is an aberration (although one that shows no sign of ending so far). But the Muslims in a position to turn in terrorists know none of this for sure, and the U.S. government is doing nothing to win their trust (indeed seems to be going out of its way to act untrustworthy).
If I were in that position, including that lack of certain knowledge, I wouldn't turn anybody in. Heck, even here in the West, I wouldn't turn anybody in unless I was certain that they were in a position to kill people (or something equally nefarious). I'd rather die than send somebody to the black hole of Guantánamo on mere suspicion or talk.