I don't care if they're very likely to commit acts if they walk free. If we don't have enough evidence to charge them, we have no grounds to hold them. That's how the justice system works.
And that is one way to deal with it, though I doubt Obama will do it because 1: Americans are unlikely to love him for freeing "potential" terrorists in America, and 2: Americans are certain to hate him if one of them goes boom.
Not saying it's the wrong (or right) way to deal with the issue, just that I don't see it happening in practice.