I'm taking the UC Berkeley class in AI through edx.com and I can see why you wouldn't want to offer credit. It would be way to easy to cheat without having some paid testing company giving tests like they do for certification exams. But it is a great resource for surveying a discipline and seeing how its taught in a major university.

I don't see any reason why they couldn't make it work for the kind of massive introductory courses freshman take. How is it worse than 500 students taught by a grad student in a state university lecture hall?

I think at least some of the opposition is by professors worried about protecting their jobs and not wanting to lose the comfortable path they have trod so many years. But they will likely have no choice. I'm not sure how it will end up, but academia can no more avoid the massive changes in our world brought on by the internet than bookstores or newspapers could. If higher education was a free market and not so massively subsidized, the changes would have already happened. Given a choice would anyone want to pay the $200,000 full price for a four year university degree in say something like English lit if it were coming out of their own pocket?