If there is nothing actionable that the person has done, then it strikes me as unfair if not bordering on illegal to deny them the ability to travel by air by placing their name on said list.

Furthermore, wouldn't finding out they can't fly because their name is on the list tip off those suspects to the fact that they are being watched by the government, or else direct them to other options (booking tickets with their middle names, using initials to book, using a fake ID, etc.), which not only does nothing to really prevent them from flying, but also lets them know we might be after them?

Some might claim "letting them know Big Brother is watching" would be a deterrent to their potential terrorist activity, but I'm not so sure.

Now, there may be "undesirables" on the list, people the US doesn't want to fly into (or around) our country, but other than a border control issue preventing them from entering in the first place, I wonder why they'd be on the no-fly list domestically yet not detained for their acts when detected.