the inconvenience mostly (but not only) relates to the person or people in the seat(s) in front of the parents. Someone who paid for a seat on a long-haul flight should be able to recline if they desire and be comfortable in a seat they PAID FOR and not have to sit straight up the whole time in order for a child to be comfortable in a nonseat they did NOT pay for (or BARELY paid for, as it were...)

that's one of the major aspects.