Of course they can. All they have to do is to draw that arbitrary line you draw at "sentience" ... somewhere else. After all, it's merely your opinion that a human being is defined by "sentience" (whatever "sentience" is ... do you cease to be a human being when you are asleep?). Someone else can have a different opinion. What makes yours more valid than his?
Certainly, and I can see the potential for abuse by drawing lines. However, I am pragmatic about this issue. At the end of the day, our society needs to come to some type of consensus as to what type of abortion, if any, is acceptable. The consensus now seems to be that partial-birth abortion goes too far, as does "life begins at conception." As usual, the solution is somewhere in the middle. My definition (life begins at sentience) is much closer to "life begins at conception" than it is to "life begins when the baby is fully borne."