Why do the Tralfamadorans not believe in free will?

From the text:

'It would take another Earthling to explain it to you. Earthlings are the great explainers, explaining why this event is structured as it is, telling how other events may be achieved or avoided. I am a Tralfamadorian, seeing all time as you might see a stretch of Rocky Mountains. All time is all time. It does not change. It does not lend itself to warnings or explanations. It simply is. Take it moment by moment, and you will find that we are all, as I've said before, bugs in amber.'

'You sound to me as though you don't believe in free will,' said Billy Pilgrim.

'If I hadn't spent so much time studying Earthlings,' said the Tralfamadorian, 'I wouldn't have any idea what was meant by "free will." I've visited thirty-one inhabited planets in the universe, and I have studied reports on one hundred more. Only on Earth is there any talk of free will.'