As a writer, and someone who has done a lot of reading by writers about writing, I think that most likely PTA is not pleased with this draft getting leaked. Only a small handful of people have actually read it, so perhaps he has the perspective to see that it's not that big of a deal--most people will be evaluating his work by the final film, as it should be. Most writers, in discussing writing, say that they need to feel like they can create a space in which to take risks throughout the first draft. The thinking can be, "I know this is shitty but I'll write it down anyway, because I need to get something down," or it can be, "I know that this MIGHT not work, but maybe it's crazy enough to work--I won't know until I write it down and look at it with fresh eyes in a few weeks' time."
Many writers do not show their first draft to anybody at all; it serves as a base to write a second draft, which they would only share with a few trusted people. THIS draft of The Master (really, the Untitled Paul Thomas Anderson Project) is certainly at the presentable stage, so not a first draft, but an early draft, most likely in that "only share with a few trusted people" stage. This is why I doubt that PTA is happy about strangers reading it.
I haven't heard of any writer, no matter how experienced/awarded/lauded, who has said that a good track record allows them to feel all that confident about their new work. I do hear of writers who, after decades of writing and recognition, are still emotional wrecks who are completely doubtful of their ability to write anything good. Being a well-known and respected writer comes with its own unique burden. Hell, people REVERE PTA, so the possibility of being evaluated by an early draft might well be terrifying--as if people might think, "I thought you were the one who was going to save cinema; you're gonna try to save cinema with this?"
That said, it's out there, it's available, and I read it to educate myself as a writer. I don't think it has spoiled the movie for me, and I gained a lot of insight into the writing process by reading it. So much so that I am starting to read a lot more not-yet-produced scripts and early drafts. There's so much to be learned as a writer. I don't expect PTA to feel honored that I read his early draft--he might well hate knowing that fools like me are reading his script at this stage. I did try, though, to make it clear in my mind that I was not evaluating the final film. I wanted roughness, I craved the incomplete! I got it, and through it saw the unique beauty of a work in progress. It's teaching me how to build a script.