Over the past couple years social networks have been used by news and other sites to allow commenting by visitors.
The basic idea was that if a visitor comes to your website you would prefer some type of authentication and since your visitor may have found the link from a social network like twitter why not use that service to let them comment.
Its a pretty easy system for webmasters to implement. Normally the CMS platform like WordPress that they use provides a plugin that seamlessly automates the login process.
Unfortunately many websites are now doing away with the practice.
The reasons? Well in the UK many sites are being sued over their visitors comments. Some visitors that post comments have even found themselves in jail.
In the USA its probably more about marketing and why allow another service to keep tabs on your visitors when you can force them to sign up directly and then track and advertise directly to your visitors.
Huffington Post was one of the first American companies to restrict logins and they have been joined by at least a half a dozen of the largest content providers… most likely within a year we will see most companies doing away with social network logins all together.