If abortion is to be a crime, as many would like, then somebody has to be punished for the crime. The assumption is that this would be the doctors, exclusively, while the women undergoing the abortions get off, I assume, with a stern lecture.
That is not, however, how it usually works in criminal law. In criminal law, if you are planning to rob a bank, and I drive you to the bank and wait while you are inside doing your business, then I am not an innocent party. I am a bank robber, just like you, and if you shoot a guard while inside, I'm a murderer, too.
So if abortion is murder -- the reason we're banning it, supposedly -- then why would not the women who delivered their fetuses up to slaughter be equally guilty as the physician who actually does the deed? That's how they do it in South America.
Whether abortion should be treated as murder is in fact a rather contentious issue among pro-lifers, who are united not in believing that abortion is murder but that it is a violation of the right to life. Not every violation of the right to life need be treated the same way as murder, and there is in fact a long history of treating abortion as a distinct and relatively unique way to violate the right to life, although the popularity of this view seems to swing wildly through time. It is one of a number of contentious issues, like the death penalty, related to the question of the right to life and how it can be violated.
But let's take the view that abortion is, in fact, murder in a straightforward sense, which does seem a very common view at present. Why (setting aside purely prudential reasons of politics) is it that there's a tendency to want to shield women from legal consequences in this matter? I don't think there's actually much mystery here; you can ask a lot of pro-life activists, and, while you'll get a lot of confused and odd answers as well, the coherent reason you'll get most often is that there is should be a presumption that women are pushed into it. The analogy would then not be to people driving the get-away but to people who are coerced into opening the safe. No one thinks that such people should be treated as accomplices to the robbery, even though they facilitate it. Indeed, so disinclined are we to do so, where there is no specific reason to think the person complicit on other grounds, that we simply presume that the person in question has no guilt whatsoever, despite actively contributing to the robbery. This is a very widespread view among pro-life activists, and it seems to be getting more common: that abortion is not usually an expression of liberated woman's choice but is instead usually the result of others restricting a woman's choice for their own convenience. The people who on this view are like the get-away car drivers are boyfriends or parents who push for the woman to get an abortion, where they exist. The women are like the people forced to open the safe: yes, it's possible that the person opening the safe is actually in on the criminal aspect of the act, but barring clear and definite information on it, that person is presumed to be a victim rather than an accomplice. Of course, there are purely practical considerations, as well, like the hard sell point. And, believe it or not, there are quite a few people in the pro-life camp who, while they regard the pro-choice position as ultimately untenable, nonetheless sympathize with it, and are willing to make what limited concessions they can in that direction.
In any case, abortion is a tricky kind of case to analyze, precisely because it joins into one a number of things that are, outside of cases of abortion, rarely found together. Not, of course, that they are never found together -- for instance, it's a surprisingly common view, for instance, that it should be illegal for women to drink while pregnant, due to fetal alcohol syndrome, and some of the same issues arise there. It doesn't help that this is an Any Stick issue -- both sides, besides giving arguments that are worth taking seriously, also give forth a continual stream of bad arguments because any stick is good for beating their opponents. This leads people to try to minimize the fact that it really is peculiar case, where a whole bunch of moral considerations suddenly come together that usually are easy to keep apart. I would suggest that what is needed is not analogies to other crimes, but a careful inquiry into the nature of this particular kind of case to show either (in the pro-choice case) that there is no morally consistent way to make it illegal or (in the pro-life case) that there is a clear and appropriate legal solution.