At quick glance their page does have a login button, which you can't implement on a static site. It also has social network buttons. These are probably all useful features for them and their audience
Indeed, though that's extra work. One can also just send people to the social media site when they click on the site's button, afaik they all have dedicated pages for sharing content (they want you to share stuff on their platform). Don't need to import their javascript and run it on every visitor's system just to have links to their sites.
