If I am not mistaken, and I am sure someone will correct me if I am, a 16" barrel needs to have the gas port in the traditional shorty position. Total guess, but the gas port looks to be approximately 8 inches from the chamber area. Then the front sight is put out to about 14 inches or so away from the chamber, in order to achieve the longer sight radius on a shorter barrel (which is what this design is all about). If they skip the gas port at apx 8" and extend the gas tube all the way out at the front sight base apx 14" from the chamber, on a 16" barrel, you would not get the correct gas pressure and it would lead to a much less reliably functioning AR15. The shorter barrels need the gas port to be placed closer to the chamber to get the appropriate amount of pressure to run the system properly.
BTW, there are probably a ton more technical terms and phrases that should be used when explaining this, but I don't know what they would be. I'm sorry if I did a hatchet job while trying to explain this in the simplest terms... Simple is about all I understand anyway
edited because I type very poorly