First, Tyson of Autochthonous says this:
The most common form of the pill is the combined oral contraceptive, which contains estrogen. Estrogen can in some cases lead to nasty things like blood clots, and it is a risk factor (in the Pill, anyway) for cancers of both the breast and the cervix; so physicians generally like to check up on you once a year to make sure the estrogen's not done anything particularly brutish.
To which Will Duquette adds:
Also, different "Pill"'s have different strengths and side-effects. Deciding what's appropriate for the woman in question is non-trivial.
I suspected as much, but I of course am not any kind of pharmaceutical expert (and no, having spent a year-and-a-half hawking generic liquid pharmaceuticals in a telesales job does not count).
I do recall hearing, a while back, about employer-provided health care plans that covered Viagra for men but not The Pill for women, which struck me as being about as sexist a policy as I could imagine. But there are almost certainly good reasons for The Pill being RX-only, at least in its current formulation. (Now, that doesn't mean that I don't think an OTC Pill would be a good thing, if they could develop such a thing.)
As long as I'm on the subject, the idea of pharmacists refusing to dispense The Pill even to women who have prescriptions nauseates me. It's none of their damned business, and in any event, I happen to know several women who are well beyond their child-bearing years who take The Pill because its hormonal contents somehow alleviate the more unpleasant symptoms of menopause. And I'd really like to know if those same moralizing pharmacists are dispensing Viagra to men while they're denying The Pill to women.