Anything you pitch into a wort does not fly around in the air. Unless you're throwing dregs on your ceiling, the bugs in your wort are only in your wort and whatever your wort touches. They don't fly out of your fermentation vessel, they have no way to defy gravity. It's the same reason a sanitized tin foil cover over your carboy works, it keeps things from falling in there. That's all an airlock does, it keeps things from falling in. Jamil had a mini-rant in a show about this, makes sense to me, I believe him.