It's true that it is "dumb luck" that Mr Duncan didn't become symptomatic during his long trip to Texas, but even if he did, I think we need to be more careful about how we define "exposures".

I would agree with your point if Ebola were a truly airborne disease like measles, where sitting on the same plane or walking through the same airport terminal could expose you to the disease, but Ebola requires contact with bodily fluids, which dramatically reduces the number of people directly at risk.

If Mr Duncan became feverish and had body aches on the plane and I was sitting in the row behind him, say, the risk appears very low. If he was vomiting or coughing on the plane and possibly creating infected aerosols, that changes the risk profile.

That said, out of prudence, I'm sure the authorities are reaching out to lower risk or contacts-of-contacts to alert them to self-monitor for a few weeks, just in case. That's what the Texas authorities seem to be doing. I'm not sure what the airlines are doing at this point.