I bet it's just because the recipe is marked as soy-free. See the asterisk next to soy-free and then next to the milk choices? So long as your soymilk is unsweetened, I'm sure it will work fine in the recipe.
this is what i was thinking, i just wanted to make sure the soy milk wasn't gonna break down funny or something. i've had my crock pot for 3 years and have used it like.... four times