And you know this risk was low because? Are you saying that you had more information on this than those agencies tracking the reentry? Otherwise you are speculating.
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.'
NASA's policy is to avoid uncontrolled reentry when the chance of killing someone is greater than 10^-4.
How valuable is that? We can determine that using the concept of the "statistical value of a human life". For example, the NRC assigns a human life a statistical value of $9M when determining of the cost of some nuclear safety improvement is justified. This is similar to the value used for determining if traffic safety improvements are worthwhile or if medical procedures are justified.
Multiplying, this is saying that it's unacceptable that the cost of a reentry be as a little as $900. This seems quite unreasonable: the cost of avoiding uncontrolled entry should often be much higher than this. And indeed, if you look at what NASA actually does, it often gives wavers of this requirement. This tells me the requirement is of dubious legitimacy, existing more for PR purposes than because it's good policy.
I doubt the Chinese care about the political optics enough to waste their time avoiding such a low expected cost outcome. Nor should airlines cancel flights to avoid something like that either -- the cost far exceeds the expected benefit.
