This.
It's no glitch or bug, just a little fun factor from the developers. You might wanna check Rooster Teeth channel on youtube with regards to this matter. Hope it helped.
Its a glitch, yes they can use it as an twohand weapon and it's real good, downside is that you can not remove it later and its probably heavy.
As it works as an twohands weapon they probably thought of including it but dropped it as it's so large it look stupid, however they did not think about the order followers to pick up function.
Another nice glitch: you can fill your follower with gear and then drop stuff on the ground and order them to take the 20 armor sets.
However don't ask them to pick up stacks as they destroy all but one item.