Because absolute slavish devotion to moral consistency isn't a virtue when we're using arbitrary terms and categorizations, often used in different contexts.
The reason an outside 3rd party causing the death of fetus* isn't the same as the mother causing the death of the fetus is the same reason a mother
Because it's a bundle of cells that is completely dependent upon your body for biological survival, you get more say in what happens to to it.
No, I am not going to go through every single law concerning the unborn to look for "inconsistencies" before I care about what is being done now that has actual real world consequences.
*And yes pedantics among us I am fully aware that it's only a fetus after the X so and so week and if it comes for a specific reason of Southern France otherwise it's Sparkling Embryo until it enters the atmosphere and then it's a zygote but I don't care...