Unfortunately I think this is the case with most airlines. I rememeber flying with my wife from London to LAX a few years ago, with one airline I cannot remember which now. She had been visiting family in London so had booked a return flight at different times to me. Because I had to work, I joined in the UK later. Anyway, like with you, when I tried to arrange to get a seat with her it wasn't allowed! I insisted also that I would like an aisle seat, as I have a bad back. This was also denied, with the darn rule book shoved into my face. At home I have a best rated mattress topper
bought here and so I am used to having my back, spine and shoulders soothed when I sleep. Well, my my my. You can imagine how displeased we were. I ended up cramped in between two rather large 'gents' from Newport. Chugging back cans of lager throughout the trip. My back was in agony. Not a flight that can be forgotten in a hurry.