What is your evidence for that statement?
I've read the opposite, i.e. that natural miscarriages are not nearly as likely to predispose a woman to breast cancer.
One hypothesis I remember reading, years ago, was that elective abortions cause pre-cancerous cellular changes in the breast, the mechanism being the extremely abrupt withdrawal of pregnancy hormones.
Natural miscarriages, on the other hand, may involve a different, or perhaps more gradual tapering off of hormone levels. Research in this area may be hard to come by, however ... it's so politically incorrect that merely inquiring into the subject could end a researcher's career.
My wife's doctor told her this and I also heard it on Teresa Tomeo (sp?) radio show several years ago, I don't recall her source.