At the climax of the first movie, it turns out the code is to detonate the bomb. The timer runs out, and nothing happens. Since the Big Bad wants the bomb to go off anyway, why not have the bomb go off when the timer does?
A weak reason is given in that it was the only way to make sure it was in position so YMMV.
Considering that Taha was going to shoot the bomb to downtown with a missile as the countdown would run out, it was a wise precaution.
The government actually didn't know Taha could do that, so they either covered all their bases to exclude the mere possibility of it being used against them, or they're just a bunch of idiots.
Realistically it makes sense that anyone who wants to live wouldn't allow the bomb to stick around if they knew it was counting down. In actuality it probably makes less sense that there *wasn't* a plan for the bomb to be shipped somewhere else.
