Even though, users may tweet a link, they are not the owner of the content of those links.
IANAL, but I have a feeling, its NOT OK (both legally and ethically) for a commercial to redistribute the content (without taking prior permission/licensing, that is), deviating the original creators from revenue/traffic et al.
Article explains how they only redistribute teaser text and and that reaction from media companies has been positive— they all want to get in on Flipboards's recommendation engine.
IANAL, but I have a feeling, its NOT OK (both legally and ethically) for a commercial to redistribute the content (without taking prior permission/licensing, that is), deviating the original creators from revenue/traffic et al.