You haven't demonstrated that atheists have
any basis for morality.
It's not subjective when there's a higher authority involved. Moral equals obedience to God and immoral equals disobedience.
Here’s prominent atheist
Jerry Coyne’s editorial in USA Today to explain why atheists can’t ground free will.
Excerpt:
"And that’s what neurobiology is telling us: Our brains are simply meat computers that, like real computers, are programmed by our genes and experiences to convert an array of inputs into a predetermined output. Recent experiments involving brain scans show that when a subject “decides” to push a button on the left or right side of a computer, the choice can be predicted by brain activity at least seven seconds before the subject is consciously aware of having made it. (These studies use crude imaging techniques based on blood flow, and I suspect that future understanding of the brain will allow us to predict many of our decisions far earlier than seven seconds in advance.) “Decisions” made like that aren’t conscious ones. And if our choices are unconscious, with some determined well before the moment we think we’ve made them, then we don’t have free will in any meaningful sense."
If you don’t have free will, then you can’t make moral choices, and you can’t be held morally responsible. No free will means no morality.