I read a news article last week that said the government thinks the next terror attack(s) will come from radical "survivalist" type people ala Timothy Mcveigh(sp?). Not necessarily survivalist groups, but ultra radical members within these groups.
This doesn't justify these steps in my opinion, but it shows maybe why they are thinking this way. Maybe why, according to a recent post, the "terrorists" in the warning posters were all white guys.