If no-one paid to pre-book seats, the prices would go up, Iam. The pricing model for cheap flights works on the basis that, whilst some passengers only want the absolute basics - the flight and a seat somewhere on the plane - others will want or need the extras - pre-booked specific seating, baggage allowance etc. If no-one paid for the extras, the airlines wouldn't carry on offering the basic price+extras prices - they would go back to the old model where you paid a much higher price but all the things that are 'extras' now, were included in the price.
The new pricing model has opened up air travel to many more people - it's more affordable than train travel in plenty of cases, especially if you only want the basics - but this only works for the airlines if enough people buy the extras.
"Paying to prebook a specific seat does not exempt a person from the moral responsibility to move if a child needs to be seated with their parent."
What nonsense, AdjustableWench - why should I take more responsibility for a child being seated with their parent than that parent had taken?
If you were at the supermarket, and had bought a really nice sandwich, and were accosted by a parent who demanded you swap it for their child's plain bread roll, because their child 'needed' the better sandwich (a sandwich they could pay for themselves, but had chosen not to), would you hand it over? I very much doubt it!
I would never dream of expecting any other individual to take more responsibility for my children's safety and wellbeing than I was prepared to take myself.