The reasoning is this - if Comcast builds out to the entire city, they're building out to highly profitable areas and to less profitable (or even unprofitable) areas. They do the build into the market with the understanding that they will make money on average, looking at the whole city, even if they lose money in some neighborhoods. Now Centurylink comes in and builds only in the expensive neighborhoods - well, guess what? They can offer cheaper rates in those neighborhoods, because they don't have to offset their losses in the poor neighborhoods. If they snipe away enough Comcast customers, eventually Comcast has to pull out entirely because they're losing money. At that point, who serves the poor neighborhoods? I am not a Comcast fan, but they absolutely have a point here. Competition isn't fair if one provider is being required to serve the whole city, but the other is not.
The solution, of course, is municipal broadband.