He is obviously surprised to see a Wookiee in captivity. This might explain the Imperial Officer's surprise in A New Hope when he asks Han and Luke disguised as Stormtroopers, 'where are you taking this thing?'. The expectation from The Revenge of the Sith however is that the Empire are pretty meticulous about killing Wookiees off rather than imprisoning them, as the Imperial troopers take particular care after the battle on Kashyyyk that all the Wookiees are dead.
IMPERIAL PRISON STAR WARS SERIES
There seems to be a theme developing here and it is further extended in the first series of the Star Wars Rebels cartoon when we find Ezra Bridger and his companions rescuing captured Wookiees from the spices mines of Kessell where they had been transported by the Empire to work the mines. Next, in The Return of the Jedi, he is delivered as a prisoner in chains to Jabba the Hutt by Princess Leia Organa in disguise as the bounty hunter Boushh and we again find him in a prison cell when Han Solo is freed from the carbonite. Firstly, he has the binders put on him on the Death Star in A New Hope, then he is a prisoner on Bespin and kept in a cell when the heroes are betrayed by Lando Calrissian. In the Original Trilogy of Star Wars films Chewbacca seems to spend half his life either being a prisoner or pretending to be a prisoner. Wookiee Prisoners in the Star Wars Movies.