Because the animations support the assumption. On each trigger pull, the hammer is pulled back before striking the pin and firing the weapon. The standard hand cannon animations imply that the weapon is double-action, while the animation for The Last Word implies that it is single-action. In both cases, the hammer is shown to be striking the pin causing the weapon to fire. In the case of the Rhino, if the hammer does cycle in double-action mode, then it completes the action with a half-trigger pull, and the gun fires with a full trigger pull with no further action from the visible hammer (as the firing mechanism is internalized). Here's a video explaining how the Rhino functions. https://www.youtube.com/watch?v=6gQiXoiMl4c&feature=youtu.be&t=59