Rent Capture and Government

What explains a graph like this?

Station domination is blanketing a Metro station with your ads. It is nearly four times as expensive to dominate Capitol South as Metro Center or Union Station, even though Metro Center is a larger station (four lines instead of two) and Union Station has the most traffic of all the stations in the system (Metro Center is second; Capitol South is 29th).

As government has patronage to disperse, securing that patronage becomes very valuable. Capitol South is used by representatives, and they have trillions of dollars to spend. In 1794 James Madison wrote, "I cannot undertake to lay my finger on that article of the Constitution which granted a right to Congress of expending, on objects of benevolence, the money of their constituents." Yet the extent to which it happens is shown by how much advertising costs at Capitol South and Pentagon.