Probably because the main requirement to being a priest is that person administers the beliefs and teachings of a particular religion or cult. Dragon Priests* for instance seemed to be masters of Destruction and also powerful Conjurers. Their beliefs were altogether militant, though, and their gods were not nice by any stretch of the imagination. For this type of priest, soul trapping presents no ethical dilemma whatsoever. A priest of Mara or Stendarr would have very different views and skill sets, but would still likely employ enchanting of some kind, if only religious enchanting of altars and amulets, which may not even involve conventional methods like soul gems. It's possible this kind of enchanting uses some other mechanic not represented in the game. Something like the runic enchanting of ESO, or something more spiritual and direct, like a blessing from an actual deity.
*In the specific case of the Dragon Priesthood, while it's logical to assume they'd be expert Enchanters, they were not personally responsible for the enchantments on their masks and staves. According to lore, these were crafted and enchanted by the Dovah themselves and passed down to their most faithful human followers. While it may seem odd to think of a creature with a body type like a Dovah crafting and
enchanting things, it's essential to remember that the Thu'um can be much more nuanced than it's presented in the game.
