@hawkeye10,
It was illegal for him to give her the drug, it was a controlled substance and she was a minor. That was one of the felony charges against him. It doesn't matter whether she had used drugs or alcohol before. Giving a female a drug and alcohol, to reduce her ability to resist sexual advances, and then having sex with her, despite her protests, resulted in his being charged with rape by use of drugs.
That's why the D.A. makes such a big deal of it. Those were the indictments handed down by the grand jury. He not only raped her, by having sex with her despite her protests, he used drugs to facilitate that rape.
The current D.A. makes a big deal of it because there is a public perception that all Polanski did was statutory rape, because of her age. What Polanski actually did was much more serious than that, and the D.A. is simply making that clear.
Even with an adult woman, giving her drugs and alcohol can affect how able she is to knowingly consent or protest or resist, and it would be a factor in a rape. You aren't suppose to use physical force to overpower the woman's resistance, and you aren't supposed to use chemicals or drugs to do that either. If you do, that's rape.
And he did these things with a 13 year old child. That's a very big deal. A big criminal deal.