I don't know why they implemented it that way. And the only reason I am blogging about it is that I broke my self-imposed moratorium on playing around with new-fangled Internet things and started messing around with Ning.
For example, let's say Case, as a large and diverse institution, wanted to experiment with social networking to do things like court alumni for money or provide faculty with buzzword-y ways to interact with their students. Several requirements that would pop up is to make sure that the social networking site falls under the "Case brand." That means a
case.edu URL and being able to make the website look like a Case-branded website. Both are easily accomplished. Another requirement would be to make sure users logged on to the social networking site with their Case credentials. We've got an OpenID provider; we just wish we could use it.
Yep… the entire point of me writing this post is to whine about wanting Ning to have OpenID. I did poke around their developer forums and found these two threads which sound promising albeit in a someday-in-the-future sort of way.
Is Ning planning to implement OpenID?
We don't have any plans to adopt Open ID. Also it'd be very tricky to replace the Ning ID login system with one using OpenID, but it is probably possible. The implementations I can think of that wouldn't involve the rewriting of many components would require some creative / hacky designs :)
An API that allows transparent SSO integration of external applications with your social network is still on roadmap but it is not planned for the next few releases.