The layer containing airport labels is overwritten by the layer containing city labels. See for instance
http://www.somebits.com/multimap/map.html#10.00/37.2992/-121.9487
http://img.skitch.com/20111220-cm16jw4sfpnd61xi4n5i7nywni.jpg
At z=10, "San Jose" writes on top of "SJC".