I'm less concerned with the Foundation being sued (because we are protected in most cases, both by our ability to respond to takedown notices and our educational purpose) and more concerned by the vulnerability of content reusers to suit. Our policy on free content is not to protect *us* - if that were the case, we could just request permission to use whole troves of content and be done. The policy protects those who, through our license, reuse our content for their own purposes. They are potentially much more liable to suit and this liability for them violates our goal to assemble a completely reusable base of knowledge.
The best way to protect those who wish to utilize our content under its license is to ensure that we adhere to it - or change it. The best way to ensure our compliance is by guarding the insertion of non-free content - not laboriously deleting it once its eventually noticed.
Nathan