You make it sound like it's a chore for him to make a Batman movie.
I bet you no matter who directs Batman 4, we won't get it until around 2015 or 2016, 4-5 years after Batman 3.
By that time it would be easier for the general public to accept another directors style or another Batman. If Nolan and co. decide they just can't get away from the character then the films might be more successful and the alloted time gives them time to refill their creative juices with new and fresh ideas.
From a career standpoint it is a limiting decision. Time spent on a Batman film is time spent not doing a creatively different film. In terms of career highlights Memento and TDK are the most powerful works of Chris Nolan. Memento established him as a sort of contemporary Hitchcock, an intelligent director who can craft an engaging story with the shocking twist at the end. TDK took his career to the next level. It wasn't just a summer blockbuster that made a ton of money. Critics were saying things like "redefined the genre" "The Godfather II of comic book films."
He could have just left after TDK since the film served its purpose for him careerwise. Which is why I give him a lot of credit for coming back and working out a deal that didn't require heavy handed negotiation. But the third film comes on a decade spent doing Batman. For a director who keeps talking about doing things fresh, new and challenging it doesn't seem likely that he would continue with Batman.