Sophie Deveraux said that witches from New Orleans practice Ancestral Magic. She stated that if a witch leaves New Orleans their access to magic is cut off, they cease to be a witch. However, Finn left New Orleans but was still able to practice magic.
It is unknown if he was still able to practice magic due to channeling Esther and Mikael.
Josephine had enchanted St. James Infirmary to be a place where all magic is void, similar to the The Magic Purification Spell. However, vampires such as Marcel were able to enter and survive, in theory, he should have died as the vampire species is the result of Esther's spell.
However, this could have been due to the fact that the Magic Purification spell stripped Mystic Falls of magic, the spell Josephine performed could of just stopped the practicing of magic.
However, this wouldn't make sense as the spell voided Hope's magic suppression bracelet.