Dissident, consider the times it was, the place it was, and the situation it was. Drugs and alcohol were consumed regularly, and would have been consumed without Samantha; and Samantha would have consumed them elsewhere too (at least in the Hollywood environment). I see nothing unusual with Samantha consuming alcohol and drugs in that place and situation, even with asking for them, or with Polanski offering her them. Because of that, I can't believe that he literally "drugged her to rape her". I can believe that he dismissed her "no" and proceeded on nonetheless. But I don't think the drugs were part of a sinister plan to rape her.

Now, was it a wise thing to do?? Probably not. But neither of them could know how this would end up changing both their lives. And, as the other post by Newfie exposes, it really isn't like this whole situation of underage sex and drugs was uncommon in the 1970s (so that to both of them it could have been quite normal, and they didn't ask themselves if it was wise by Interpol standards).