My guess is, because they didn't hijack the plane.
They were found with shaved bodies carrying box cutters within a week of the 9/11 attacks -- that didn't get them enemy combatant status. It's not surprising that acting highly suspicious on a plane didn't get them in more trouble.
There's also the possibility that the man I spoke with jumped the gun in connecting the arrests in San Antonio with the incident at Dallas/Ft. Worth. But I'm convinced his story about the men on the plane is genuine.
In normal circumstances, that would be the answer, of course.
But Padilla is being held even though he didn't do anything (yet), and he's apparently not being charged with a crime. It's not at all clear what the U.S. government's standards are for such arrests.
As for the four possible hijackers, if the San Antonio guys were two of them, I'm curious who the other two might have been.