From the author note of (ironically) one of her RPF missions, "Downer:"

"Don’t you dare try to tell me you’re a fan. Yeah, you’ve read Journals. I can see that. But did you really understand it? [. . .] This is the kind of image that got Frances taken off Kurt and Courtney in the first place [. . .] It’s not even like he was someone in a book or a movie – Kurt Cobain was a real person, depressed, maybe, drugged, sometimes, but not deserving of this defamation of his character. And as for what you’ve done to Courtney . . . well, yeah, maybe Courtney wasn’t meek and mild, but she wasn’t like this. And you have to accept that they loved each other. Maybe it wasn’t some Elizabeth and Mr Darcy or Romeo and Juliet written-in-the-stars thing, but they chose each other and you have NO RIGHT to talk about their personal life like you know what was going on."

It's a bit of an extreme example, to be sure, but the main point is still there. Just as the PPC doesn't flame authors--because hurting the feelings of real people lies far outside the realm of literary critique--neither do we want to imply knowledge of others' private thoughts and personalities and present it as fact. That, too, could lead to hurt feelings and a misrepresentation being taken as true.

...that by "allow RPF" you mean "Why we don't spork bad RPFs."

It's a bit complicated, but I think the biggest factor is that the entire concept of the RPF is kinda weird to begin with. We base our humour on pointing out inconsistencies with a canon work and raging at the sight of mangled masterworks. We can't really reproduce that effect with fics that deal with real-life people. I mean, Real Life doesn't have canon, right? Furthermore, it feels really odd to write dialogue for someone who's... well, real and alive. It's almost like putting words in their mouth.