Why be good?

[Cinjin]

If there is no God, then there is no hell; and if there is no hell, then there are no ultimate, eternal repercussions, good or bad, for how we live out our mortal lives. Of course, atheists insist that people should be "good without God."

But why? If God does not exist, why be good?

Where is your authority to claim that if a god doesn't exist than there is no eternal repercussions?  
Says who?  You?  Who are you again?

As far as I'm aware, atheism means no belief in a god.  True, most atheists don't believe in any form of afterlife, but I have met a small few who are open to the idea of an afterlife without god.  

My only point here is that you created a non-sequitur in your OP simply by assigning the afterlife to a god.  There are many, both believer and non believer who would dispute your small mindedness and your unsupported assertion.