If there are two seats together in one row or one empty middle seat in another, why would BA not fill the middle seat with a single passenger to preserve the two together?

For the HBO passenger, BA are complying with the contract they entered in to. In short, the HBO passenger played and lost because if they didn't want to risk being allocated a middle seat, they could book a plus fare or pay for seat selection.

Disappointing The Sunday Times is competing with the Daily Mail on non-news. Soon we'll have no newspapers to choose from in the lounge.