I'm guessing that even some guy that looks at various websites and makes claims of “something needs to be done” with regards to cops or the government could be suspected as a terrorist. (Like Freerepublic!?)
Although it sounds like they had specific threats of him wanting to behead a cop - hence the surveillance.
Of course he wasn't shot for being a suspected terrorist. He was shot for lunging at the cop with a large knife in his hand.
If there was a specific threat, why no warrant?
Why no overwhelming force?
Why question him at 7 AM at a bus stop?
That is not how the FBI operates