Before the passing of the Fugitive Slave Act, northerners could view slavery from a distance. To them, it was an issue that had to be dealt with by southerners. Abolition was pretty far from most people's minds still.
The Fugitive Slave Act made it a illegal not to turn in people suspected of being runaway slaves, so it forced people to take a stand; either by choosing to follow the law, or to disobey it and engage in a criminal act. Either way, someone was taking a stand on slavery in a way that many people did not have to do in the north prior to 1850.
Hope this helps!