Why is burden of proof shifted in case of allegations?

It has been my experience, in the USA, that if a woman is d, she must be the one to prove it was and not consensual . I don't know about India, though. As much as I hate the intense interrogation process a victim must go through, I do understand it is for a good purpose. I stand by the "innocent until proven guilty" mentality--although it sucks that many rapists get off because its often hard to prove in a "he said, she said" scenario.