They control their name and branding pretty well. It's not like they're going to open up a ton of classes and let everyone get the degree.
As for an online degree, it's a joke. Really there is only so much you can learn on your own. You need the interaction with people in class or group settings and the resources you pay that much money for.
Aside from the resources, people go to these schools because of the heavy recruitment from the top firms in the country and your access to alums. Guess what? Having taken one class, online, in the extension school is not going to give you access to those recruiters and you won't have any way to connect with the alumni.
Stanford posts a lot of its lectures on itunes for free. You can listen and learn if you want, but don't expect to become an expert in those subjects
-Rudey