The potential issue isn't with the child flying.
The issue is that the child is flying with adults who don't appear to be her parents.
For better or worse, airlines, TSA, and immigration/customs are looking out for parental abductions and human trafficking.
Some countries won't let you in without both parents present and/or a signed/notarized letter from the absent parent (Canada often does this). So, the airlines will sometimes ask about it before boarding an international flight (assuming you check in at the desk).
The easiest questions for TSA to ask the kid are those about who they are, who they are with, and where they are going. Not that those questions are any guarantee the kid isn't being forced beyond his/her will, but at least they aren't turning a completely blind eye.
Even if flying unaccompanied, sometimes the agents will ask these questions. I think that has more to do with not showing an ID, though (ie, they want to know the kid isn't old enough to need one).