Flying SQ economy soon. Looked at a seatmap and saw a less-than-full cabin.

Reserved the aisle and window seats in very normal seats (no extra legroom, not up front, nothing special), leaving the middle seat empty.

Checked yesterday and I now see the middle seat filled by a stranger, even though there are a number of other aisle, middle, and window seats available (but no two side by side that we could move to).

The call center refused to consider either contacting the other traveler or manually changing their seat assignment.

I understand I don't have a right to an empty seat between us, but I do have Star Alliance Gold status and it seems very odd that Singapore Air would place someone between us when many better seats are available.

It will certainly keep me from booking SQ again for quite a long time, if ever.

Your thoughts on why they would intentionally punish the other traveler and simultaneously irritate a frequent flyer?