Until now you have been very civil in your argument, Darwyn, but the last sentence was a condescending, even more so because SD astonishment is understandable. If 'you can't know [that] he isn't a target' then you know that he might be target. So there is no certainty that the president is a target nor that an attack is imminent. Of course the Secret Service may also act on a mere possibility of an attack, but as soon as you admit that an attack on the president was not an imminent certainty, but only one possibility of many, than you have to deal with all the arguments, reasonings and surmises presented earlier.
I don't know if logic is fully applicable to analyse the situation, but as I see it you have constructed an argument by contradiction from 2 statements;
1. The Secret Service knows with certainty that an attack on the president is imminent.
2. If the Secret Service knows with certainty that an attack on the president at his current location is imminent, they will always move the president immediately and under all circumstances.
If you assume these two statements to be true then it follows that the president has to be moved, which is in contradiction to the actual events, so one one of the statements has to be wrong. This is as far as the logic goes, which of the two statements is wrong is a matter of another debate.
You preference is for the first statement, but even this statement can be negated in several ways. One way, your way, is to say that the Secret Service knows with certainty that an attack on the president is NOT imminent. Another way would be to say that the Secret Service doesn't know with certainty that an attack on the president is imminent.