No one is blaming the pregnancy on the fetus, and I'm not sure why you assumed that to begin with, considering that obviously doesn't even make sense.

The part you seem to be misunderstanding is that pregnancy itself IS an act in which a fetus is literally using the body of the mother; the fetus is physically attached to the mother's body and is using her body to live. If the mother does not consent to her body being used for this pregnancy, then she has every right to stop it from continuing. The fetus does not have the right to use the mother's body, and is dependent on the consent of the mother to allow it to continue to be there in the first place. Even if a grown adult person was attached to you and using your body to keep themselves alive, you would still have every right to decide that they cannot continue to use your body if you don't want them to, even though they will die without you. Again, no one has a right to use another person's body, for any reason (that includes pregnancy), and only you have the right to decide who can or cannot use your body, for any reason (including pregnancy). If you don't want a baby to use your body for pregnancy, then you have every right to stop them, even if that means they die, because they did not have the right to be there in the first place, and they definitely do not have the right to override your consent.

Abortion is justified because the consent lies with the mother whose body is being used, not with the child who is using her body.