Journalistic standards. Original reporting and honest attribution are longstanding journalistic values. If your site publishes aggregated content, you will need to separate it from your original work, or restrict our access to those aggregated articles via your robots.txt file.
Uniqueness is mentioned several times in the guidelines. In your situation, a big concern would be getting reported by the originators of the material.
I certainly would want to duplicate anything that Rupert Murdoch owned, eg. He's been particularly outspoken about news copying. Understandably, other content originators might also be upset if they see you using their material without permission.