"From what I've read, there is currently very little overlap between the regions of Time Warner Cable and those of Comcast [1]. If this is correct, how could the merger allow either firm to raise the price?"
I read an article on this the other day, can't find it right now. The objection is based on the merged company gaining too much power over (potentially competing) content providers, not over their own customers. I think the gist of it was that Netflix (or any similar company that serves content requested by ISP customers) might be able to thwart "extortion" by individual smaller ISP's, because if Netflix stood up to the ISP and ISP responded by cutting off Netflix service, Netflix would still be able to survive and derive sufficient profit by serving the population of ISP's in other areas. If a single ISP serves too much area/population, however, Netflix's negotiating power falls rapidly. In Time Warner/Comcast case, the single merged company would control so much area that as a practical matter Netflix would have to do whatever they demanded, because Netflix would lose too much money if it tried to fight the extortion and lost ability to serve all Time Warner/Comcast customers. (I believe crux of this argument depends on antitrust laws preventing individual ISP's from acting in concert against Netflix, which laws would not apply against Time Warner/Comcast as single entity.)
So basically the argument is that the TimeWarner/Comcast merger would create a company that has too much power over its competitors, leveraging its function as last-mile ISP to gain unfair advantage over content providers it competes with in providing its own content.
I read an article on this the other day, can't find it right now. The objection is based on the merged company gaining too much power over (potentially competing) content providers, not over their own customers. I think the gist of it was that Netflix (or any similar company that serves content requested by ISP customers) might be able to thwart "extortion" by individual smaller ISP's, because if Netflix stood up to the ISP and ISP responded by cutting off Netflix service, Netflix would still be able to survive and derive sufficient profit by serving the population of ISP's in other areas. If a single ISP serves too much area/population, however, Netflix's negotiating power falls rapidly. In Time Warner/Comcast case, the single merged company would control so much area that as a practical matter Netflix would have to do whatever they demanded, because Netflix would lose too much money if it tried to fight the extortion and lost ability to serve all Time Warner/Comcast customers. (I believe crux of this argument depends on antitrust laws preventing individual ISP's from acting in concert against Netflix, which laws would not apply against Time Warner/Comcast as single entity.)
So basically the argument is that the TimeWarner/Comcast merger would create a company that has too much power over its competitors, leveraging its function as last-mile ISP to gain unfair advantage over content providers it competes with in providing its own content.
I did find the article, and there's a lot more to it than what I wrote in previous paragraphs: http://www.vox.com/2014/5/6/5678080/voxsplaining-telecom