The absence of densely populated locations in the middle of the country reflects the difference in terrains between the coasts and the middle which is more mountainous and less city like. Steph Abegg also makes the point that the amount of locations are proportionate to the state population and as California has the largest state population, it is common knowledge that it would have the most Starbucks. California's high number of Starbucks Coffee shops also reflects the nature of the state, as one filled with contemporary franchises.
The second diagram is a more graphic representation of every location in the country and clearly shows the huge dispersion of the shops around the coasts. The North of the country has the least amount of Starbucks, perhaps suggesting something about their style of living.
Sources: Steph Abegg: 'Starbucks: A West Coast Phenomenon' (stephabegg.com)
James Davenport diagram (ediblegeography.com)