I haven't seen this mentioned so far, but perhaps the shooting of the helicopters fuel supply was a de facto quarantine? Not enough time for ritual slaughter, just pop a few rounds into the neighboring helicopter whilst taking off.

As for the lack of zombie clean-up, we have to presume that the zombies are at least semi-concerned with self preservation, an open area like that would be too risky to clean up, there could easily be people watching from the aircraft control tower etc.