Well, it's not like anyone stays in slavery by choice. Incidents in the Life of a Slave Girl takes a hard look at the three laws that keep the institution of slavery going: (1) Slave-children always follow the condition of the mother, so plantation owners just added to their wealth by raping and impregnating their slaves. (2) Slaves could not read, write, learn, or teach, so they'd never find out that things could be different. (3) The Fugitive Slave Law, which made it illegal for Northerners to help runaway slaves. What's the point of taking this legal look at slavery? By living in a country governed by these laws, Northerners implicitly allow slavery to continue.
When Jacobs writes that the novel is her “testimony,” she uses legalistic terminology to put her country on trial.
The Fugitive Slave Act of 1850 brought slavery to the North, since it made northerners responsible, by law, for returning fugitive slaves to their masters.