Atheism as hedonism?

I recently saw some photos of the so-called "atheist buses" in London that carry the first atheist ad: "There's probably no God. Now stop worrying and enjoy your life".

I wonder what were the criteria for selecting this phrase as the slogan of atheism. Correct me if I am wrong, but the message of this slogan is that religious people do have philosophical concerns and philosophical worries, while atheists are a bunch of hedonists that they do not want to worry about philosophical uncertainties. Actually, the main reason to become an atheist according to that slogan is to have fun.

Gross. Sartre, Nietzsche and the rest of the existential movement pantheon will turn in their graves. Someone must remind to these gay dudes that atheism is closer to existentialism than hedonism. The realization that there is no god is a lonely brave act that should lead every individual to a personal existential crisis. The individual then gives a new meaning in his life by redefining his personal values and struggles to work for the well-being of the other lonely souls out there.
If that's a bit too much to ask from the nihilists, how about just not hijacking atheism in the name of hedonism?