The thinking was that if we prosecute the woman as well she would hold back on identifying the abortionist and the case against her would most likely be lost as well. She will have to deal with God, but not with the legal system.
Then that is something to be decided on a case by case basis, like with any other crime. Often someone complicit will roll on others and cut a deal. The idea of them being held liable under felony murder rule could actually HELP get the abortionists rather than the other way around just based on how it is used in other cases.