Ben from Wistia here. I can't speak directly to why SEOMoz doesn't use all of the social sharing buttons available. That being said, I can give a little insight as to how the Social Sharing features work within Wistia so that you can make an informed decision for yourself.
First, there are basically two classes of Social Sharing buttons that can be enabled. The first class is Wistia based buttons that we build directly and directly affect the video. These include:
- Embed - provides an iframe based embed code to embed on third party websites
- Stats - provides a link to a stats summary page for the video
- Email - a mailto: link that allows someone to send the link to your page
- Logo - an image and link that points back to your site
The other class is third party buttons for various social services. In these cases, we implement the button code as specified by the various social services. These include:
- Twitter
- Facebook
- Digg
- Reddit
- Tumblr
- Stumble Upon
- Google Plus
All of the third party buttons are designed to share URLs, not individual pieces of media, such as videos. As a result, when you click the Facebook Like button on a Wistia video, you are actually Like-ing the page.
For all of these buttons as well as the Email button, we wire the URLs to the first page where the video is embedded. This way, even if your video gets embedded by various people (via the Embed button), all social activity will still point back to you.
As an example, let's say you embed a video on your website with the embed button and the Twitter button. Someone comes along and uses the embed button to get a copy of the video which they embed on their blog. One of their readers comes along and views the video. After enjoying the video, they click the "Tweet" button. The link (and text if configured) that gets Tweeted out will point back to your site, not the blog on which it was viewed. This allows your videos to get more social reach beyond just your website.
Again, I can't speak directly to SEOMoz's social strategy. It may be that they already have social buttons on their blog posts in other locations and feel that buttons on the video would be redundant or distracting. That being said, I hope this helps explain a bit more about how the the social sharing buttons work and gives more insight as to when you would want to use them and when you wouldn't.
Let us know if you have any additional questions.