It is probably correct that most atheists support the right to abort a human baby.
That doesn't mean the logic supports them. It may or may not. It certainly does not mean that atheism requires support for abortion. There are atheists who are pro-life.
Most libertarians argue for abortion, but their beliefs don't come out of libertarian theory, and there are libertarians who are pro-life. Libertarianism does not require support for abortion, and, indeed, you can fashion a libertarian argument against abortion (I basically did above). Similarly, atheism does not require support for abortion, and you can fashion a non-religious argument against abortion (again, as I did above).
Just because most theists are against stealing does not make stealing a religion-only issue. Secular law is quite capable of handling that issue, and any other rights-violation, consequential issue.