Why be good?

[Jenny A]

Can atheists justify, according to atheist principles, why they believe it is "wrong" to pollute oceans, cut down rain forests, or hack into someone’s bank account and steal their life savings? If the stronger members of the human species engage in such behaviors in their pursuit of dominating the weaker members, and if there is no God and therefore no transcendent, prescriptive moral law given by God to guide us into knowing what is right and what is wrong, then on what grounds can atheists legitimately oppose such behaviors?

You really like strawmen don't you.  There is no such thing as atheist principles.  Atheism is not a philosophy.  It is a state of being in which a person does not believe in a god or gods.

Individual atheists myself included might have various reasons for why they believe theft and murder are wrong.  Some may also be environmentalists, but not all. 

I personally believe somethings are wrong and somethings are right out of a combination of gut instinct (evolution is at work there I'm sure), empathy for others (evolution again), social training, and reason.   If I were to state my operating rule when in doubt it would be this:  choose that course of action which you would have required if you were able to write the rules for humanity before your sex, race, nationality, abilities, etc. were known.

The justification is simple.  Morals are there to allow people to live together in the most mutually advantageous way possible.

Doing so would be intolerant and would have the net result of the atheist forcing his morality on others -- the very thing atheists object to in the first place.

First of all, once again not all atheists object to religious morality.  That generally isn't why atheists are atheists.  (hint it's that little matter of the complete lack of evidence for god).

So what is the justification for imposing my morality on others?  None, I don't and I can't.  But society can enforce it's morality and does so in the form of laws and social interactions.  In democracies those laws enacted based upon societies common sense of morality.  When one religious group attempts to take control of this process the results tend to be bad for society as a whole.  Secular morality is therefore preferable.

And, as it happens, I do object to strictures imposed by religious persons when the only justification for it is my god says so. Let me give you an example.  Thou shalt not kill is pretty much universally accepted as a good rule, advantageous to society as a whole. Thou shalt not eat pork is not.  Nor is thou shalt not take the name of the lord in vain or thou shalt have no other gods before me. My objections come in the form of political participation and/or appeal to the First Amendment.