Guys, guys, do some research before making wild generalizations.
Please read: http://en.wikipedia.org/wiki/FreeTrack
FreeTrack has its own interface, plus works with SimConnect and FSUIPC and can emulate mouse, keyboard and joystick (better than TrackIR can).
From both a legal and ethical perspective, interfaces are there to be used by everyone, to prevent false monopolies such as this. If you're running an "IBM compatible" computer you should be well aware of this.
FreeTrack can use the Nintendo Wii Remote camera which runs at 100fps and uses less CPU than TrackIR. As well as the Playstation Eye camera that runs at up to 120fps.
Data is output at 120fps for a 30fps camera, so smoothness is not an issue, only responsiveness. And high responsiveness is not essential for head tracking, especially in flight sims.
FreeTrack tried to support TrackIR hardware, effectively helping NaturalPoint, but they wouldn't allow it.