It probably should fall under fair use, but without more information it is really hard to tell whether it is legal or not. The more important question, ultimately, is whether the news provider is likely to take legal action, because legal or not, if they do take legal action, you are probably shut down anyway.
If you are only posting their content, then the use of it is probably going to be seen as parasitic. In other words, you are stealing their traffic by getting indexed for the same content, deriving your ad revenue, and then passing the traffic back to them that they should have gotten in the first place, but with viewers who are marginally less likely to convert. (this is not my view, per se, but likely the news provider's view)
If you are aggregating news from multiple different sources, and are not providing full text, and are crediting and linking, then I think it would be very difficult to argue that you were not simply exercising fair use, and providing a news aggregation service, but I would look over the TOS of any provider you were scraping to be on the safe side.
To be on the safe side, I would have an abuse@domain email set up, and check it fairly frequently. If you get pestered, be polite, but ask the news provider to clarify their Web 2.0 policies, and ask them if they have similarly complained to G00gle (or other large aggregation services) to be removed from them as well. This will give them pause, because they will not want to be removed from G00gle, but if they insist that you take their content down, then it sure looks like they are discriminating against you.
