Dear Larian, I did not find a single instance where i was put off by the models/graphics b/c of the 360° rotation. So, I do not understand why ristricting (or "fixing" the above) the camera rotation at all.

;-)
I always assumed it was to cut down on the amount of work for the level designers, particularly with interiors. The two walls closest to the player (bottom of the screen) are usually lacking the detail compared to the top two (which will usually be riddled with things to interact with).
When screenshots of their game pop up, I'm sure they want people to see this:
instead of what's on the opposite side of that room:
..as the latter has the potential to hurt sales.