It is likely spelled out in the airline policy as well. If your child is old enough to face either way then yeah it's considerate to put it forward facing especially on a red eye. But it is quite common go FAs to insist they have to go forward. It's not a problem in economy plus or F. Anyway I found there was enough give if you tilt back the infant's seat a little the restraint might tilt the child up a little but the person in front can still move back some.