While Tally and David ride towards the city, David tells Tally a bunch about the world, especially about the Rusties.
For instance, David takes Tally through the empty wasteland created by the white orchids (romantic).
And they go through a Rusty city, where David explains that the Rusty civilization was destroyed because (1) they were dependent on oil (ahem) and (2) someone invented a disease to infect oil (which sure gives us some ideas for a fun weekend).
That's not the story that Tally learned in history class, but maybe the city doesn't want people to know that all civilizations are vulnerable. Hint, hint.