As your history teacher loves to tell you, studying the past is the only way to understand the present and guess about the future. (If your history teacher hasn't told you that, then you are the lucky exception to the rule.) So, when David shows Tally some ruins and tells her that all it took was one big shove to destroy the previous civilization, well, that makes us a little nervous about the survival of the whole pretty world. And as authoritarian governments often do, Tally's city controls information about the past so that people behave themselves in the present. And that's why, in Uglies, the past may be a clue to the present, but sometimes it's hard to know exactly what went on in the past. (Also because there was a lot of fire, which tends to make old books go bye-bye.)
In Uglies, Tally's present is our future—which makes her adventures partly our fault.
Uglies shows the Specials trying to control how people remember the past (it was bad, things are better, go have a drink). Except the past can never be truly controlled and often breaks out into the present in uncomfortable ways (like in magazines).