""Why believe in a god? Just be good for goodness' sake,"

According to the Houston Chronicle, this December The American Humanist Association will run an ad campaign where they will write, ""Why believe in a god? Just be good for goodness' sake," on the side of busses.

As you can well imagine, certain sectors of society are up in arms about it.

The humanists' entry into the marketplace of ideas did not impress AFA president Tim Wildmon.
"It's a stupid ad," he said. "How do we define 'good' if we don't believe in God? God in his word, the Bible, tells us what's good and bad and right and wrong. If we are each ourselves defining what's good, it's going to be a crazy world."

I worry about people who need an extrinsic reason to behave morally. If the fear and punishment factor were to be removed from these people, are they telling us that they will stop behaving morally? Are they saying that the only reason they behave morally is because they fear their punishment in hell? Or if it is not punishment, why do they feel that they need an external authority to tell them how to behave?

The Objective Standard has a good article that addresses the root of "good" behavior for non-theists.