Look closely at the advertising and you'll probably find that they do say that. There's probably some fine print to the tune of "nationwide coverage claims based on 89% of US population". That means that they don't claim to serve the least-economical 11% of the country, as determined by population, which is of course a huge land area. They could exclude all of Kansas, Nebraska, and the Dakotas and still meet that claim. (In actuality, they probably do serve, KCK, Wichita, Omaha, Lincoln, Sioux Falls, Fargo, and Bismarck -- just not the hundreds of miles of small towns and farms in between.)
Their Web site is quite honest about this (much more so than most carriers' coverage maps that I have seen):
Garrett A. Wollman | As the Constitution endures, persons in every snipped-for-privacy@csail.mit.edu | generation can invoke its principles in their own Opinions not those | search for greater freedom. of MIT or CSAIL. | - A. Kennedy, Lawrence v. Texas, 539 U.S. 558 (2003)