Duh - it's going to come down in one place. The odds of "any" one individual being in that exact place is negligible. On the other hand, where it comes down is under it's flight path, which is a predictable series of points on a line, ordered in time. If you (or your airplane) happen to be in one of those points at the same time, your odds go way way up (for a few minutes). The flight in question was no doubt going to be under the boosters flight path. You know this ahead of time, and you can obviously delay with minimal consequence (minimal compared to an airliner being hit with falling debris, anyway.) In that situation, it'd be negligent to say 'screw it, the odds are in our favor, lets just ignore it.'