What's funny is, depending on which side of Frisco you are on, you do have that choice. In general, if you are west of the Tollway, you can get both services. Several other cities in north Texas are like that (Watauga, for instance, as well as Keller, north Fort Worth, far North Dallas).
What you are seeing, at least in Texas, is the historical division between what used to be GTE and Southwestern Bell. Back when those companies were regulated utilities, you are exactly right: the markets were divvied up between the two. Generally, Southwestern Bell got the big cities and GTE got the suburbs and rural areas. Inertia and a general unwillingness to rock the boat means that situation persists, though you can see some areas where it has melded.