This may be one of the great misunderstandings of the open source era, and a big part of the FOSS-open source split.
Commercial open source, some think, does indeed "appropriate returns from the commons." I, on the other hand, believe such operations are a net benefit to the commons.
The experience came four years ago. I was asked to help launch an open source site for politics. I recommended Drupal for its scalability, but the company failed, in part because we could not develop the site quickly enough.
Since Drupal.org only provided a directory of possible assistance, we wound up dealing with an Indian outsourcer my partner was familiar with.
Their claim of expertise was false. I spent months trying to explain what we needed, and each iteration of the software grew worse. We finally got things rolling after another consultant turned us on to the new, stable Drupal code base.
By then it was too late. The business model was flawed in any case. I wound up blogging about politics, reading political blogs and summarizing their messages, but traffic was never more than a trickle, and interactivity was virtually nil.
Should I get another opportunity, I will know more of what to do. I'll be able to get the help I need through Acquia. I'll pay for it, but I will have an effective site in a short period of time, and technical management will be done by techies, not journalists.
A commercial arm retains a project's market share, and its development momentum, so that add-ons gravitate toward it. I am certain that Drupal sites launched in 2009 will be light years ahead of those from four years ago.
They have to be, because much has happened since 2005. It's no longer enough to support blogs, or diaries, or to do them in a scaled manner. Now you have to support a host of other files, and social networking functions.
A CMS system, like any system, must continually progress to stay relevant. Commercial arms help open source projects meet this competition, at the same time they provide a business model which feeds the lead developers.
It's not an appropriation from the commons at all.