Well they put the appropriate regional stock cable in the box, i can't imagine how hard it would to just attach it.
Attaching the cords takes more time, and thus money. The more times the case is opened, the more risk for damage.
I'd imagine the companies make two versions to accommodate the voltages. Then they determine how many are going to which region based on demand. That's probably when the packing people include the cable.
Removable power cords isn't due to audio nuts. I highly doubt my cable box manufacturer used a removable power cord so I can experiment with different one. Same for my TV, Apple TV, XBox, et al.
I don't know the exact reason, but there's probably several good ones, otherwise they'd all be hard wired.