The only thing I can think is that the oils come with a bit of detergent in them, so they will disperse in the water for long enough to be absorbed by the wipes, when you put them in - and once absorbed, the oil will be sufficiently dilute on each wipe not to be an issue.
Otherwise, I assume your concern is that there could be concentrated drops of the oil on the wipes, @userpmfrdh - which might not be a good idea (though I assume the oils they are using are safe on skin).