Soap and modern detergents break down fine in the soil of a leach field. Public lands resource managers aren't worried about groundwater contamination from backpackers, they're worried about surface water contamination.
Detergents are bad for fish, frogs, and assorted critters' mucus layers.
Detergents may add nutrients that feed algae to the water.
So backpackers are careful about using soaps and detergents. We don't wash dishes, hands, clothes, or bodies very well. We forego using a few grams of detergent a day to remove dirt, body oils, food waste, unmentionable waste, sunscreen, and insect poison/repellant from our bodies, clothes, and gear.
Instead we just jump our grimy selves into every lake or stream for a nice swim, proud that we aren't contaminating it with evil detergents.