It must be easier (for the developer) to just keep the shield on the arm. Also, with arrows and a two-handed weapon, the back would get crowded if a shield was back there too. People have already complained from the clipping from arrows and two-handed weapons being on the back. The shield had to go somewhere, and so it stayed on the arm.
Perhaps Bethesda could have had the arrows only visible when a bow was equipped, and a shield otherwise?
Since this is merely a graphical "issue" and one that you only notice when playing in 3rd person, it's not anything that would keep me from enjoying the game.
It's a pretty easy fix if you ask me - if you equip a bow, arrows are slung on your back (or by your side in the belt, like some archers prefer). If you equip a two-handed weapon, no shield or arrow quiver is displayed. If you equip a one-handed weapon (or magic) and shield, the shield is swung across your back when you sheath your weapon, except when in dungeons where you'd keep your shield at a more ready state.
As for the whole shield issue being bothering, it doesn't bother me at all gameplaywise, but since the thread is about carrying shields on your back, I gave my opinion.
I'm afraid that sheaths and holsters (or whatever you call the things you hold axes and maces in) were deliberately made to allow for easy drawing of said weapon in a combat situation. Shields were strapped across the back quite securely, and they would not be so easy to get off. Seriously, just think of how you secure a shield to your back.
Carrying a shield across your back is no more complicated than carrying a bow across your back, and wouldn't take longer to equip. A shield can be carried in a single strap worn diagonally from one shoulder, across your chest to the other side underneath your arm. Very much like how you carry a bow by using the string as a carrying strap across your chest.
This is the tricky part, in my opinion. How would you define "not expecting combat." You can be attacked almost anywhere, including in towns. And without distinct zones and hit detection, carrying it on your back, as it stands now, wouldn't provide any additional protection to attacks from behind than if you were wielding it.
I would define "not expecting combat" basically almost anywhere you would sheathe your weapon, perhaps except when in a dungeon. Mostly, a shield was carried on your back when traveling, so it would make sense for a shield to be carried on your back whenever you're in a town or traveling with a sheathed weapon above ground.