Flower, as a somewhat simplistic answer to you (good) observations, as a whole, the human moral code is based more on our environment than anything else. Why do you feel it's wrong to steal, even from a billion dollar co? Because you were raised in an environment that felt that way too. It really has little to do with god/no god. Just because someone chooses not to believe in a god doesn't automatically make them immoral. People who might feel inclined to 'break the law' probably fear jail/gov't punishment as much or more than fear of displeasing god. It really comes down to what a person feels in their heart (conscience).
For every one 'bad' athiest, I can show you one 'bad' christian/hindu/muslum/communist/whatever. People are people no matter what belief they ascribe to. Some people do things that the majority of society frowns upon. Always have. Probably always will.
Agnostics then set their limits based on many things - upbringing, current personal beliefs, etc. Many agnostic/atheists are very good people whose beliefs(non-beliefs?) cause them to be very respectful of life, other humans, the earth, etc. Since we don't feel that god will swope down and save us despite ourselves, we tend to try and take care of what we have - ourselves, family, the earth, etc.
We have dramatic, and yes extreme, proof of what fundimentalism can do - those people who flew into the WTC truely believed with all their heart, soul, and mind that they were doing right in god's eyes, and that they would be instantly in paradise. It would be MUCH harder for an athiest to perform such a task.