Not sure about that. If you could mix cabins, eg one way Y, back in PE, it would overall be higher yield I would have thought.
I think it can still be done domestically though with the similar inferface... I think it's just another design issue.
and dare I say it makes offers like they used to have such as "Buy J one way, get F the other" nigh on impossible to book online (in the older version you just picked the appropriate flights/cabins and it was easy).
Hope they fix this (and bring back that particular offer

) - but that's just about me.