The thing is the findings for studies on breast cancer and birth control pills are inconsistent. Take a look at this information I found on www.cancer.gov:

Some studies have shown an increased risk of breast cancer in women taking oral contraceptives, while other studies have shown no change in risk.

-A 1996 analysis of worldwide epidemiologic data conducted by the Collaborative Group on Hormonal Factors in Breast Cancer found that women who were current or recent users of birth control pills had a slightly elevated risk of developing breast cancer. The risk was highest for women who started using OCs as teenagers. However, 10 or more years after women stopped using OCs, their risk of developing breast cancer returned to the same level as if they had never used birth control pills, regardless of family history of breast cancer, reproductive history, geographic area of residence, ethnic background, differences in study design, dose and type of hormone, or duration of use. In addition, breast cancers diagnosed in women after 10 or more years of not using OCs were less advanced than breast cancers diagnosed in women who had never used OCs.

It also says: Studies have consistently shown that using OCs reduces the risk of ovarian cancer. The use of OCs has been shown to significantly reduce the risk of endometrial cancer.