I have no idea why Ben Brooks would want to do this. For starters, if your site is public there's not much stopping scrapers from removing the content and displaying it differently, especially if you have a full RSS feed. The best way to cut the amount of money they make off of your writing is to accept the contributions from readers who have submitted your articles to the service. I understand that some people object to Readability profiting from their content, but they take the money pre-contribution. Readability isn't taking 30% of publisher revenues, but they are taking 30% of your total contribute amount. It's all just fancy language but in the end, the perception makes a huge difference. I'll gladly give Readability money to help grow their platform. I pay for many of the services I use the most to help support development.
To play devils advocate, Readability should be much more transparent about what's done with revenues from sites that are not a part of the service. It might be helpful to have something like "verified sites" to let readers know who they are supporting and who they aren't. This even may have an impact on users actions and sell some publishers on the service.