because that choice could have killed other people, made them seriously ill...that's an irresponsible, childish choice.
it's not the same as allowing people to make decisions that effect only themselves. you're just waiting to hit me with "but that fetus is a person, what about their choice?" ...a fetus isn't a person. a fetus is a potential person, with no thoughts, no life yet, and if it is aborted before 24 weeks (which is a very conservative estimate, in reality it's probably more like 30-32 weeks) it is not murder, it's a woman making a choice about her life that only she and her doctor are qualified to make.
if it only effects your body, it is your choice. if it effects everyone who comes in contact with you, and everyone who comes in contact with them, it stops being personal choice and becomes public health hazard.