I feel the answer is 1)because
the waist needs to be strong as it fits the body closely and has to support the weight of the skirt or trousers and need interfacing to support it
the top of pants or skirt is finished with facing when a)extra strength is required at the waistline 2)its not possible to layer the fabric 3)a stay tape cant be used at the seams or 4)you don't want the waistband to be visible from the right side
2 answers
You asked about facing which is different from interfacing. Interfacing is used to stiffen the band so that it doesn't roll or wrinkle up.