Why Do Religious People Often Call Atheists Immoral?
1. You Have No Absolute Standards of Morality!
I think this is not true. There are some things, in my opinion, that are always wrong, regardless of the consequences. Adultery is one example. Although it is possible for there to be a gain in happiness if the other partner didn't find out and the adulterer had fun doing it, it would still be wrong. This is because of the betrayal of trust in the relationship. So it is quite possible for an atheist to have an absolute moral rule.
And of course, there are problems with the "It's moral because God" argument anyway, the most obvious being the Euthyphro Dilemma: Is it good because God commands it, or does God command it because it is good?
On one, torturing a baby could be moral if god said so, and on two, the atheist can derive the same moral conventions without God.
Also, absolute morality can be used to argue against God anyway, on the following:
(1) There are some things that it is always wrong to do.
(2) God, in the Old Testament, has done, or commanded that you do some of these absolutely immoral things.
(3) Thus God cannot be moral.
2. You Have No Reason To Be Moral!
"So, you may know what morality is, but why should you be moral all the time, as no-one is watching you!"
Here are some reasons for an atheist to be moral:
a) Duty as a reason to be moral - If you believe it is your moral duty to do something, you will believe it right to do it regardless of whether God is watching or not.
b) Compassion as a reason to be moral
c) Self-interest as a reason to be moral - Even if you break a moral conviction you have, your consciousness will haunt you about it and make you miserable. Thus, self interest can be a reason for morality.
3. The Liberal/Conservative Divide
Atheists, as a general rule, tend to be more politically liberal than theists. And atheism is often associated with liberalism by religious groups. This is one reason I believe theists may see atheists as immoral.
There is no particular justification on an atheistic worldview to argue that, say, homosexuality or premarital sex are wrong. Whereas theists often hold this conviction very strongly, due to duty to God's ethics. This leads to theists thinking that atheists are immoral, because they do not have these convictions, even though the theist thinks that they are important moral commandments.
This is just an idea of mine; it may not be correct, but it seems rather reasonable, judging on what i know of fundamentalists.
Thus concludes this discussion of atheistic ethics.