It's not the air bag reason in this case (not used), nor oxygen masks as someone else suggested (as occupying a seat); it's also not financial either as infant in seat pays a child fare (who are allowed in these cabins).
It has to be the supervision thing.
Infant in a seat is allowed on business staggered on 77W in the centre seats "paired" with the adult - in this config you can squeeze past the table so there is access between the seats.
On alternate rows of 787, there are also two centre seats, but these are not accessible from each other - the screen between them extends all the way to the seat in front, presumable to increase privacy as these seats are close and provide more table room, as the side table in these in narrower, but longer than normal seats. So in int787 the closest you could be to your under two year old is across an aisle, probably not ideal if problems during seat belts on and dangerous with a strapped-in-car-seat-child in an emergency evacuation situation.
I'd expect the F ban is for a similar reasons with those boxes round them, but I don't have experience of these unfortunately.
Even with children (over two-year olds), I don't think these seats (even on 77W staggered) are ideal for young children and they weren't design with travelling with a companion in mind.