You're assuming that the ONLY reason women want it is to avoid pregnancy. But even if that is so, aren't the low costs of birth control far more tolerable than the high costs of getting pregnant? Prenatal care, labor and delivery, post natal care, child care, etc., are all much higher costs to the insurance company, as well as the patients and society in general, than birth control.
But in the now-infamous Sandra Fluke case, immortalized by Rush Limbaugh, she was only talking about women who need those pills for medical reasons, not specifically as birth control pills. And that should be covered by insurance even by your standards.