At the moment i suspect it is both. During the pandemic with very light loads there were numerous W&B problems with passengers not sitting in their assigned seats (cabin zone) ending up with a center of gravity and resulting trim that did not really work well or unexpectedly. With light loads even small number of passengers can put the CG in a place where it shouldn't be.Not working for FR, but we did introducce a new seat-row procedure to assure that the numbers we expected match the numbers in the real cabin at least for take off as those are important for our trim setting.
And of course, we are still in a pandemic. Many countries use seating information for their trace programs and can fine airlines quite heavily if passengers were routinely not in their assigned seats.
On the other hand, of course if there is a higher price for those seats, it is in the companies interest to enforce seat assignments.