The FAA rule for the airlines is that CRS's must by placed where they "do not impede egress in an emergency", which translates as "someplace where no one will have to climb over them in the dark." Not all FA's will be aware of the rule without looking it up, if they don't fly a lot of routes popular with families they may not have had reason to know it.
On most aircraft, the rules means that a CRS must be in a window seat, but on a widebody with a center section, the seat can be placed in the center seat of the center section and still fall within the rule, since the evacuation plan calls for passengers to use the aisle nearest their seat.