They clearly recommend reserving seats.

It may be policy, likely adopted out of necessity as the lesser evil of dealing with screeching parsimonious parents, but it's absolutely unfair to those who may get moved from the seats they reserved to accommodate a parent who intentionally did this to save money.

If doing this to me involved separating me from my companion involuntarily when both seats are reserved, to accommodate someone who chose to have children but shirks paying the cost because they know "my children!" results in immediate deference, they'll have a fight on their hands.

There's also a difference when this is being done with forethought, knowing the potential effect it'll have on other pax but not caring so you can save a little cash.