Big Brother is watching.

In the end, I think it works out for everyone.

The extra seat selection fee profit that is not worth:
A) negative publicity whenever some entitled parent(s) make a stink
B) Saves on-board crew repeated headaches of shuffling families
C) Keeps other customers, happy - by not making them move seats.

They should keep the free grouped seats at the back of the plane though, since they are complimentary.