Isn't that the only reason SQ block seats though. *A seats are not available on their newer products otherwise you can pick up a seat on virtually every other route they operate on.

You are right though, makes no sense to fly a new config on this route.

Maybe a very long shot but could it be that they blocked seats after the recent mistake DL fare which routed NYC-CDG-SIN-DPS with the last segment on SQ. They must have had a huge number of bookings on this mistake fare and thus decided to shut up shop for any more free or cheap seats.

Just my 2c's as I honestly can't think of any other reason.