Consider that much of the Americas received missionaries from the church, and for centuries these missionaries worked to welcome people of many faiths into the church. One of the results of this contact is religious syncretism, a situation where people merge religious beliefs to form new ones. The early church incorporated certain dates of pagan feasts into the church calendar to ease the transition into Christianity, so this practice is not unknown in the Catholic church as well.
The crucial difference that you’ve noticed is that these syncretic faiths, in the ‘St. Michael’s Psychic Parlor’ sense, are still fundamentally polytheistic, and have adopted the outward symbolism of the church onto their pre-existing cosmologies. Their ‘St. Michael’ represents only the triumph of some sort of power, not the triumph of God. Where Catholics rightfully view Saints as servants of God who can be relied on for intercession and help, and that they ultimately exist only to serve God, syncretic faiths pick, choose, and invent in order to to glorify the particular attributes of that saint. Santeria, Candomble, and other Caribbean syncretisms use Catholic saints as almost a visual shorthand to continue their own faith tradition in a different context.
For instance, Santeria combines west African (modern Nigeria) Yoruba worship of “Orisha” (loosely, their gods and goddesses, although ‘spirit’ would probably be more accurate) with Catholic symbolism. If a particular Orisha was, say, considered to be female and to be allied with the snake, a statue of the BVM standing atop a snake would be used to continue traditional worship disguised as catholicism, and over time the BVM would be adopted into this cosmology in a completely different form that Catholics know her.
The same thing happened in Mexico with the melding of Aztec religious imagery and Catholicism. Everyone is familiar with the Day of the Dead, in which tribute is made to ancestor’s graves on All Saints’ and All Souls’ Days; this is a continuation of a festival for an Aztec female death deity, Mictecacihuatl, appropriated for Catholic Mexico.
What does this all mean? That depends. On one hand, you have to admire the resourcefulness of a group of people practicing their religion under duress from authorities seeking to suppress it; our symbolism of fish and anchor crosses, Irish penal rosaries, and others spring fro our own history of persecution. At the same time, any religion that asks spirits for favors both good and ill makes me deeply uneasy. A case in point is the latest manifestation of Mictecacihuatl, the cult of Santa Muerte, or “Saint Death.” In recent years, a veneration of Santa Muerte has arisen, most notably among Mexico City’s criminal element, and has spread rapidly amongst Mexican and Mexican diaspora communities.
This is disturbing because people pray to Santa Muerte for things they can’t pray to other saints for, like revenge, money, earthly power, harm on one’s enemies, etc. In other words, bad things no Catholic saint would condone. The Church in Mexico has openly condemned this cult as demon worship, yet popular devotion to Santa Muerte (depicted as a robed skeleton, often the Grim Reaper) has continued and even grown. Just as some gangs use Nuestra Senora de Guadalupe sometimes as a gang emblem or prison code, Santa Muerte takes catholic faith and twists it into something else entirely.
Sorry for the long post, but it was a good question!