Why believe in a god? Good campaign?

The American Humanist Association recently made a $40,000 ad campaign in which "Why believe in a god? Just be good for goodness' sake." will run alongside of buses in Washington, DC. This ad will run starting as of next week and will run through December. "The Washington-based group is wading into what has become a perennial debate over commercialism, religion in the public square and the meaning of Christmas." This was pulled from an article I found bringing attention to the ad. However, many other humanists are getting quite offended and are making it known. Some are calling it inhumane to say there is no god. What are they to believe if they are being encouraged to not believe in their own spirituality. This is absurd and very confusing. How should we celebrate Christmas without the religious history its supposed to convey? I don't think this ad is such a great idea, more than 90% of the U.S. population believes in a god therefore this ad will have no effect on them. They should drop the religious part and just keep the "just be good for goodness' sake."