Ask HN: Question related to use of RSS feeds for product

Hey everyone, I'm wondering if someone could provide clarification around how apps such as flipboard, zite, feedly, etc. are sourcing content via RSS, or other, legally. As I understand it, usage of RSS in many cases is limited to "non-commercial" use, all of the above mentioned have a commercial attribution to them.. Is it that they don't use RSS, have agreements with publishers... OR? Any thoughts appreciated.