>But as far as I can tell, there is nothing about newspapers that makes them a natural monopoly. Why should anyone regulate combinations in that area?

Not yet, but as local newspapers continue to lose readership to the internet, the will start consolidating and/or failing. After say 20 years of this, there will be only the big online national papers, NYT, WaPo and some others that serve a large niche. There will always be room for the smaller niche papers though.