On the other hand, if people do actually read it, it's clear that a lot of the descriptions being bandied about - including your own - aren't quite how things happened. Vague descriptions that he "drugged her" and even (as seen elsewhere) that he "drugged her drink" don't tally with the girl's own testimony about the less than single quaalude they shared. What is also obvious is that there is no mention of any discussion with Polanski about her age during her entire testimony. I don't think I've ever seen it explicitly proven that he knew exactly how young she was; I don't think it could be disputed that he didn't know she was underage, but did he know she was 13? As Yoink says, a lot of it comes down to "he said/she said," but I think objectively you can see how Polanski might have rationalised it in his own mind by simply avoiding seeking any clarification on that point. It's not an excuse, but I think a lot of how the first acted - at least in the early stages - would have merely reinforced the idea in Polanski's head that everythign was OK. Not wanting to dwell on the detail, but the switch from vaginal to anal intercourse is also dealt with in such a perfunctory manner that I think most people would do a mental double-take.
The bottom line, though, is that Polanski plead guilty to a specific charge, and he should receive an appropriate punishment for it. The danger, though, is that whatever that is is unlikely to the same as what he would have got in 1977.