The point of the American Girls Collection was to encourage girls to read and learn history. If you look at all the time periods chosen for each historical character, you will see a common thread--these are all times of upheaval and uncertainty in the nation's history. The age the girls are in the stories is a time when girls are trying to figure out who they are with many questions in their own lives, much like the nation's questions about what kind of country it wants to be.
Addy's stories do not glorify slavery, but make young students aware of it. For many readers, this is their first introduction to the Civil War. The Civil War period is an important part of America's history, for more than the abolition of slavery.
Are there other times in history that would serve the teaching purposes? Perhaps. Yes, the Civil War happened long time ago, but that doesn't mean we shouldn't teach it. So why make the choice of Addy starting out as a slave? Because it gives her perspective. She knows both worlds, she's experienced both, allowing her to make intelligent decisions about what she believes.