They also signed the actors who played Uncle Ben and Norman Osborn to a three picture deal too, maybe even the guy who shot uncle ben.
They just want to be sure the actor will return when Steve has nightmares about Bucky's death in all the other films.
Imagine Steve has a bad dream about seeing Bucky blow up in the Avengers movie. So they need the same actor to play him.
Plus, what if they want him to play the Winter Soldier some time later.
I wouldn't mind seeing that either (Cap movies based in WWII). The comic tried to do that a long time ago, when Cap was brought back in Avegners and then got his own comic alongside Iron Man in Tales of Suspence. They tried to bring back the old WWII Cap stories, but fans hated them or at least didn't buy them as much, so they scrapped it, and brought him back to the modern times in his stories.