Machines

When Billy is getting onto the plane that he knows is going to crash in chapter 7, Vonnegut narrates, "Tralfamadorians, of course, say that every creature and plant in the universe is a machine." (Vonnegut, 154). Why are living things considered machines to the Tralfamadorians? Do the Tralfamadorians consider themselves as machines too? Where did they get the idea of anything living being compared to machinery which is normally non-living? How can Billy except the idea that he is a machine so easily?