I wish to know- exactly what is the point? Anybody who cares about the contents has the abstract book anyway. So disallowing public discussion does absolutely nothing to keep the material secret, and is not going to make the presentations and posters more of a surprise. It's the Shiny Digital Future- when something's released online, everyone has it. So you can't claim it's to prevent stolen ideas ala Aetogate, since any paleontologist who cares has the abstract before SVP anyway. Even assuming they somehow didn't, public discussion is allowed after the meeting (or they could just attend the meeting itself), and it's not like the papers that (some) abstracts lead to are published immediately. There's plenty of time to get your own knock-off paper published, especially if you're the editor of an in-house journal so that you can speed the process up.
The only people who don't have easy access to the abstracts are those outside the paleontological community, and the media couldn't care less about basically any of the content. Even on the incredibly rare occassion the general public might care, like the Triceratops-Torosaurus synonymy announced in an SVP abstract last year, the news outlets mostly got it wrong and most people misunderstood it anyway. For last year's SVP, Google finds a whopping eight news stories that cover six abstracts, and that's due to only about five reporters. Does the SVP really think that reporters are going to scour the abstract volumes of scientific meetings or dinosaur blog posts for stories? Even things like the PepsiCo ScienceBlogs mess that caused Pharyngula to go on strike barely get noticed.
Who does notice and care though? Us, the amateur and professional paleontologists who host blogs and use mailing lists, message boards, and newsgroups. We'd gladly write up summaries of all the cool stuff happening at SVP, which would be seen by the portion of the public that is actually interested in paleontology. That's free publicity for the society, the authors, and their work. But by the time we're allowed to, there will be new stories to cover, and we won't be nearly as hyped about it. All this accomplishes is decreasing the excitement and interest in SVP's meeting among the young readers who could make up its membership in the future, and thus be paying to attend, subscribe to the journal, etc..
(Of course there's also the obligatory crowd that claims abstracts are gray literature that isn't properly citable, but they should be on my side too, because what sense does it make to have an embargo on information you consider to be equivalent to hearsay?)