Obviously, Slaughterhouse-Five is a book about prisoners of war, and it doesn't get much more confined than that. But even more, it's a book about the many, many ways people get trapped: by the army, by family, and by their own beliefs in God or glory. It isn't only the Germans or the U.S. Army who take away Billy's choices. He also finds himself caving in to the expectations of his mother, his optometry office, and even his own daughter. Billy sees very little real freedom in his life, which is perhaps why he is so eager to accept that there is no such thing as free will.
The Germans and the Tralfamadorians both take away Billy's freedom, but the Tralfamadorians go a step further by giving him the tools he needs to accept his confinement.
Even after Billy is freed from German captivity, he remains mentally a prisoner of his war experiences – until he can replace these memories with life on Tralfamadore.