But your own argument works against you. Because the difference is in whether the pregnancy itself is normal. Normal pregnancies do not normally abort spontaneously. Many spontaneous abortions occur because the woman's ovaries secrete an inadequate amount of the pregnancy hormone progesterone. If the pregnancy is completed, other hormones act to cause the breast cells to differentiate into milk-producing tissue, or to die off if they're not needed.
Differentiated cells are less likely to become cancerous.
That's why a complete pregnancy tends to lower breast cancer risk. However, if the pregnancy isn't completed, no late-term hormones kick in to differentiate the breast cells. Instead, the excess estrogen leaves the tissue in the growth-prone mode, free to grow into abnormal or even cancerous cells. Thus, more abnormal cells are left behind, and also more normal but primitive cells, which may become abnormal later on. That's why interrupting a normal pregnancy tends to increase breast cancer risk.