loosely organized band of hetergenous companies can effectively produce and support enterprise software

why the companies involved don't just open-source their software

I think that what these companies are trying to do is to smooth out some of what they perceive to be the unreliable aspects of open source software. I don't view this an attempt to extort money out of member companies, but more an attempt to solve the free-rider and other problems that corporations face when attempting to use/develop open source software. My guess is that these companies are going to end up using (and hopefully contributing back to) open source software as part of what they end up doing. In a way, you can look at it as something like a gated, for-profit version of the ASF, which employs staff developers. It will be interesting to see how this pans out.

In my mind this is a logical next step in the phenomena that Doc's IT Garage is trying to deal with -- reversing the dynamics of the vendor-customer relationship, using open source style (or more neutrally, commons-based peer production) ideas as one of the ways to do that.