For services that wish to make money from advertising, it makes sense to be open initially — but then, having attracted a large base of users, to start building walls around them.
For an economist article, this was surprisingly terse and poorly explained. That said, this is the worst generalization that I have recently read. By this generalization, all blogs should be shutting content of search engines.
This walled garden around user generated content is fairly new and was first instituted by facebook and twitter. At the end of the day, it is a poor paradigm for everyone involved, especially users and including the long term health of the company that institutes it.