Believe me, I've sent an e-mail asking for an explanation. I do think however it would be far better all round if the bsct's were on the sides in the back row downstairs (but what do I know - I'm just a passenger).
I also ran a test and tried to sit with the infant in a non bsct seat on the u/d and again got an error message.

Interestingly, if you look at the seatmaps in MMB, the cot at 64A (and in a 3 class 777 at 3E) is almost greyed out. On a laptop you have to move the scree around to actually see it exists whereas the other locations are a lot more bold.

Oh well, will be interested to see how this is defended but they can't expect to operate a 747 on a route that has a lot of VFR traffic that only offers 1 bsct in J...