I'm creating a news reader app. (Users will choose sources, but not the exact feed addresses. for example, they can choose Gizmodo but they will not see the exact url of Gizmodo's feed)

Some of sources are giving partial feeds, and user must go to their website to read the complete article. My question is, May I make a fulltext version of their feed (for example, using this service) and use it instead of their feed in my News reader categories? Am I legally/ethically allowed to do this? What about using some other service like Readability? (Readability gives HTML page instead of feed as input, but anyway, output will be clean article without any advertises or such)

Please note that some of these partial feeds may have some advertises inside, but the fulltext feed generators usually remove that advertises.