Why is Catholic-based sorcery confused with witchcraft?

Because it is witchcraft/sorcery. In many cultures, folk traditions still hang on, long, long after the people have generally converted to Christianity. Some of these things are simple home remedies or traditional practices, while others are part of the pre-Christian pagan rituals. Of course the people who practice this get antagonistic. They are strong believers in their way, and don't like to be told that they are combining pagan practices with Christian ritual. I've known a number of Catholic people who are involved in this sort of thing, from several countries, and they just don't see that their beliefs are from there folk culture. They pull the old beliefs of spirits and how to appease them into a Christian context, and use saints as substitutes for ancient gods.