Eh, the act of criminalizing something is more nuanced than that. It may be that no jury would ever actually convict the person who pushed the other out of the lifeboat, when the desperation of their situation was clear. Nonetheless, one might still hope, by pronouncing the act unlawful and subject to punishment, to make the emphatic statement that it is *never* a morally appropriate thing -- and thereby to counsel people out of doing it, to scare them out of doing it, to remove their opportunities to do it...to minimize, essentially, the possibility of its occurrence in the first instance. There are plenty of instances in which existing laws make no exception for dire circumstances, but in which prosecutors or judges or juries will quietly decline to punish certain violators. I think many (though certainly not all) people in the "no exceptions" camp on abortion laws would choose to excuse some women who aborted under some circumstances, but would prefer that the law take a strict moral stance on the issue.

For that matter, returning to the lifeboat example, I'm not aware of any murder statute that makes an explicit exception for shoving someone out of a lifeboat in a moment of desperation (or for the analogous general case) -- so I expect that the anti-choice extremists would question the appropriateness of listing an analogous exception in this instance, unless the intent were to express that such an act *is* a good and fine thing, and one for which people should even be entitled to medical assistance. And heck, since I actually do think those things, and want them expressed in the law, I don't exactly think it's a mistake to infer that intent.