So you think throwing distraught mothers, who might very well be acting on instincts and not even realized what they done, in prison is a better idea? I'm not saying it should be legal either, but as the comments note, many countries have specific laws in place to relieve the mother of responsibility in these cases without directly acknowledging what is taking place.
And there is also the suggesting that by allowing easy abortions, you lessen the number of cases where the mother will kill her child after it has been born, which by any measure really is a worse situation than an abortion.
The article itself is really all about one big source, http://www.amazon.com/Mother-Nature-Maternal-Instincts-Species/dp/0345408934.