I have made many snow/ski pants where the fly technically goes to the "top" of the pant, but those pants dont have waistbands at all. Its a feature that you will see to eliminate bulky seams on heavy fabrics, bulk for layering, or where you need to operate the open and close of a pant in cold weather or with gloves. Buttons, snaps, etc dont work in those scenarios, but are more stable and common. Another type of garment where you will see zippers going to the top edge are womens skirts and pants, because they're hips are wider than their waist, more so then mens', and if a zipper comes loose, it will stop itself as it descends to the wider section of the hip. These are also secured at top with a hook, button, etc...
But ultimately, as I said in my very first comment, the exposed portion if a zipper can be between 3/8" and 5/8", depending on how its sewn. A raw zipper trim is about 7/8" wide in total, so lets give the benefit of the doubt, and say 1" of a zipper is in play here, both tape and teeth. An entire waistband in the case of these specific pants can measure anywhere from 29"-43". Lets say the average is 36", just for this scenario. That would man that less than 3% of the entire waistband is interacting with the body. Thats moisture, outside climate, all attributing to how the pant fits. 97% of the waistband is contacting the body, that is fluctuating in body weight and movement. And its nylon, which absorbs moisture (sweat, vapor, precipitation) because it is naturally hydrophyllic, even with DWR, and will affect its long term memory (how its shaped and if it can retain its shape) and durability, because nylon fibers swell and dont shrink again until after wash. The zipper centered at the front of the fly, attributing to less than 3% of surface, cannot alter these things, and will not protect the material from responding to them. The fabric will take on shape and alter from basic wear, and unfortunately, it will GROW, changing the fit. Wear these fishing in the summer. Sweat in them for 3 days. Measure them before and after and tell me im wrong.
Hope this helps.