It was a purposeful decision. This will later become a combined image with five characters, and her facing this direction with the sword in her left hand will create a better composition. It's not absurd to have an off-hand sword anyway.
Wow, Glitch, you are kind of being rude, you know... the artist explained that the reasons were for composition of the picture (to make it look nice and pretty), and that they weren't claiming that it was her dominant hand or anything -- that's why Kanaru said it was "off-hand" (aka with her non-dominant hand). Kanaru never brought up Skyward Sword or claimed you were wrong or anything.
Kanaru, this is a gorgeous piece. I am especially envious of the folds in the skirt and the lighting!