Wow – I was going to write a research paper on exactly this issue when I studied copyright law last semester: http://www.theatlantic.com/business/archive/2012/01/over-aggregators-pay-up/251496/
The legal issues are complex for aggregators. An aggregator, which by definition copies a copyrighted work, is committing a breach of copyright. Ordinarily, however, providing an RSS feed would seem to provide an implied licence to that breach (ie there is consent). That implied licence may be modified by an express licence, such as a Creative Commons licence.
I never ended up writing the paper, but I did notice that Creative Commons prevents uses that are for ‘commercial purposes’. What this means has never been tested in a court of law. It would seem that a news aggregator who profits from advertising is in breach of that licence.
(There are many other complex copyright issues that arise from the issue)
