"Act surprised" is a strange choice of words. Like people who care about spoilers are only pretending to react things as they happen?
I'm as spoilerphobic as they come, and I think the BOOKS DO NOT EXIST rule is a great one, as there is lots that can be gleaned simply from what the readers do and don't find it necessary to tiptoe around.
That being said, the post preview thing is a quirk of the site's design, and it's great if people want to be conscious of it when they're posting about major revelations in popular works, but I don't think anyone can be faulted for not doing so.
And expecting a 2nd season thread to be free of 1st season spoilers is simply asinine. Was anyone even asking for that? It's not spoilers if you decide to pick up a story midstream.