tl;dr;fh
I just thought it was interesting that the president went to a site that has been featured on CNN for jailbait pictures, is widely popular among atheists, and is an easy access for pornographic images. Not to mention, if you peruse the comments in many of the threads, there is a strong anti-America presence on the site.
I'm not saying Romney and Co aren't snakes and weasels, but if people wanted to shout about this, I think there's plenty of ammo to use.
Oh, and here's the real reason for the AMA:
https://my.barackobama.com/page/s/reddit