Well they must because of all the crying and complaining that will ensue if people post anything.
Although I'm sure there will be those who won't use the thread
I hardly think you can begrudge people being upset if people post things about Dawnguard in the General Discussion Forum, considering it has a big ol' "DO NOT POST SPOILERS IN THIS BOARD!" thread stickied at the top.
Posting in the Cheats and Spoilers Forum is a little different, but the spoilers should still be avoided in the title. I never understood the people who post a thread that gives a major plot point away in the title followed by "spoiler." It's a little late to be warning people about spoilers after you've already told them the spoiler in the title.