Where are you going with this? Are you second-guessing this guy? Ok, whatever: I bet he wants the decision back to make it over again. But in the mean time, what purpose is this line of thinking serving? Is it so difficult to believe that through a series of mistakes (some forgivable, some not) these guys got themselves into a position where one of them made a decision that this guy was the kind of threat he'd been cleared to kill?
I guess for you it is.
> why did they not stop him from boarding the bus?
We've already established that these guys made mistakes. What more do you want? There's my answer: they let him board the bus because someone made a decision that, in hindsight, might not have been the right answer. Usually they let people go in situations like this because they think the rewards -- usually having the tailed take them someplace that increases the value of the tail -- outweigh the risks.
How about this: maybe they didn't think he was a suicide bomber until he ran? Unfortunately, when two people aren't talking and each are reacting to the other's hidden motives, the probability that they are both wrong goes way up.
/jordan