I don't know why the cover what they cover. I just know that where my wife works has switched insurance companies a number of times and there's never been even the slightest problem getting her BCP covered. They might be looking at her records and seeing that it's a medical necessity, but if they are they all did it on their own without even talking to us about it.
HMO's often cover them without a medical necessity issue attached such as endometriosis.
But insurers that don't shouldn't be made to do so under the false premise that this is descrimination based on gender because they don't perform similar functions. If a pharmaceutical manufacturer comes out with a drug that reduces spermatogenesis (production of sperm) and the insurer covered that, then I would say that BCP should be covered for women to avoid pregnancy.